Como chamar página de erro?

Advertência: org.apache.catalina.core.StandardHostValve@10418b1: Exception Processing ErrorPage[errorCode=500, location=/error.xhtml]
javax.servlet.ServletException: Context is already active
	at javax.faces.webapp.FacesServlet.service(FacesServlet.java:422)
	at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1534)
	at org.apache.catalina.core.ApplicationDispatcher.doInvoke(ApplicationDispatcher.java:787)
	at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:649)
	at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:485)
	at org.apache.catalina.core.ApplicationDispatcher.doDispatch(ApplicationDispatcher.java:454)
	at org.apache.catalina.core.ApplicationDispatcher.dispatch(ApplicationDispatcher.java:350)
	at org.apache.catalina.core.StandardHostValve.custom(StandardHostValve.java:474)
	at org.apache.catalina.core.StandardHostValve.dispatchToErrorPage(StandardHostValve.java:671)
	at org.apache.catalina.core.StandardHostValve.status(StandardHostValve.java:358)
	at org.apache.catalina.core.StandardHostValve.throwable(StandardHostValve.java:323)
	at org.apache.catalina.core.StandardHostValve.postInvoke(StandardHostValve.java:220)
	at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:328)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:227)
	at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:170)
	at com.sun.grizzly.http.ProcessorTask.invokeAdapter(ProcessorTask.java:822)
	at com.sun.grizzly.http.ProcessorTask.doProcess(ProcessorTask.java:719)
	at com.sun.grizzly.http.ProcessorTask.process(ProcessorTask.java:1013)
	at com.sun.grizzly.http.DefaultProtocolFilter.execute(DefaultProtocolFilter.java:225)
	at com.sun.grizzly.DefaultProtocolChain.executeProtocolFilter(DefaultProtocolChain.java:137)
	at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:104)
	at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:90)
	at com.sun.grizzly.http.HttpProtocolChain.execute(HttpProtocolChain.java:79)
	at com.sun.grizzly.ProtocolChainContextTask.doCall(ProtocolChainContextTask.java:54)
	at com.sun.grizzly.SelectionKeyContextTask.call(SelectionKeyContextTask.java:59)
	at com.sun.grizzly.ContextTask.run(ContextTask.java:71)
	at com.sun.grizzly.util.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:532)
	at com.sun.grizzly.util.AbstractThreadPool$Worker.run(AbstractThreadPool.java:513)
	at java.lang.Thread.run(Thread.java:619)
Caused by: java.lang.IllegalStateException: Context is already active
	at org.jboss.weld.context.AbstractConversationContext.activate(AbstractConversationContext.java:301)
	at org.jboss.weld.jsf.WeldPhaseListener.activateConversations(WeldPhaseListener.java:110)
	at org.jboss.weld.jsf.WeldPhaseListener.beforePhase(WeldPhaseListener.java:84)
	at com.sun.faces.lifecycle.Phase.handleBeforePhase(Phase.java:228)
	at com.sun.faces.lifecycle.Phase.doPhase(Phase.java:99)
	at com.sun.faces.lifecycle.RestoreViewPhase.doPhase(RestoreViewPhase.java:113)
	at com.sun.faces.lifecycle.LifecycleImpl.execute(LifecycleImpl.java:118)
	at javax.faces.webapp.FacesServlet.service(FacesServlet.java:409)
	... 28 more
----- Root Cause -----
java.lang.IllegalStateException: Context is already active
	at org.jboss.weld.context.AbstractConversationContext.activate(AbstractConversationContext.java:301)
	at org.jboss.weld.jsf.WeldPhaseListener.activateConversations(WeldPhaseListener.java:110)
	at org.jboss.weld.jsf.WeldPhaseListener.beforePhase(WeldPhaseListener.java:84)
	at com.sun.faces.lifecycle.Phase.handleBeforePhase(Phase.java:228)
	at com.sun.faces.lifecycle.Phase.doPhase(Phase.java:99)
	at com.sun.faces.lifecycle.RestoreViewPhase.doPhase(RestoreViewPhase.java:113)
	at com.sun.faces.lifecycle.LifecycleImpl.execute(LifecycleImpl.java:118)
	at javax.faces.webapp.FacesServlet.service(FacesServlet.java:409)
	at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1534)
	at org.apache.catalina.core.ApplicationDispatcher.doInvoke(ApplicationDispatcher.java:787)
	at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:649)
	at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:485)
	at org.apache.catalina.core.ApplicationDispatcher.doDispatch(ApplicationDispatcher.java:454)
	at org.apache.catalina.core.ApplicationDispatcher.dispatch(ApplicationDispatcher.java:350)
	at org.apache.catalina.core.StandardHostValve.custom(StandardHostValve.java:474)
	at org.apache.catalina.core.StandardHostValve.dispatchToErrorPage(StandardHostValve.java:671)
	at org.apache.catalina.core.StandardHostValve.status(StandardHostValve.java:358)
	at org.apache.catalina.core.StandardHostValve.throwable(StandardHostValve.java:323)
	at org.apache.catalina.core.StandardHostValve.postInvoke(StandardHostValve.java:220)
	at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:328)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:227)
	at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:170)
	at com.sun.grizzly.http.ProcessorTask.invokeAdapter(ProcessorTask.java:822)
	at com.sun.grizzly.http.ProcessorTask.doProcess(ProcessorTask.java:719)
	at com.sun.grizzly.http.ProcessorTask.process(ProcessorTask.java:1013)
	at com.sun.grizzly.http.DefaultProtocolFilter.execute(DefaultProtocolFilter.java:225)
	at com.sun.grizzly.DefaultProtocolChain.executeProtocolFilter(DefaultProtocolChain.java:137)
	at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:104)
	at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:90)
	at com.sun.grizzly.http.HttpProtocolChain.execute(HttpProtocolChain.java:79)
	at com.sun.grizzly.ProtocolChainContextTask.doCall(ProtocolChainContextTask.java:54)
	at com.sun.grizzly.SelectionKeyContextTask.call(SelectionKeyContextTask.java:59)
	at com.sun.grizzly.ContextTask.run(ContextTask.java:71)
	at com.sun.grizzly.util.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:532)
	at com.sun.grizzly.util.AbstractThreadPool$Worker.run(AbstractThreadPool.java:513)
	at java.lang.Thread.run(Thread.java:619)

<error-page>
    <error-code>500</error-code>
    <location>/error.xhtml</location>
  </error-page>

<h:inputTextarea value="#{exceptionManager.stackTrace}" rows="25" />

@ManagedBean(name = "exceptionManager")
public class ExceptionManager {

	public String getStackTrace() {
	
              //método responsável por retornar a pilha e exibir na tela
	}