Fix errors when redirecting to error page when response is already commited

Some errors persist in excpetion handling mechanism (see NXP-3428), when mock faces context created for error handling is not properly removed from current context.
For instance:

1. exception hadling catches an error, for instance:

Caused by: ClientAbortException: java.net.SocketException: Connection reset
at org.apache.catalina.connector.OutputBuffer.realWriteBytes(OutputBuffer.java:358)

2. error is handled correctly and put in nuxeo-error.log : [org.nuxeo.ecm.webapp.shield.SeamExceptionHandlingListener] Created mock faces context for exception handling

3. Response is already commited => ERROR [org.nuxeo.ecm.platform.web.common.exceptionhandling.DefaultNuxeoExceptionHandler] Cannot forward to error page: response is already commited

4. mock faces context created previously is not cleant up correctly => we get additional errors like:

javax.servlet.ServletException: org.apache.jasper.JasperException: An exception occurred processing JSP page /login.jsp at line 206
...
Caused by: java.lang.UnsupportedOperationException
at org.jboss.seam.mock.MockApplication.getELContextListeners(MockApplication.java:93)
at com.sun.faces.el.ELContextListenerImpl.contextCreated(ELContextListenerImpl.java:70)
...
java.lang.NullPointerException
at org.apache.catalina.connector.Request.removeAttribute(Request.java:1354)
at org.apache.catalina.connector.RequestFacade.removeAttribute(RequestFacade.java:514)
at org.jboss.seam.mock.MockExternalContext$2.removeAttribute(MockExternalContext.java:274)
at org.jboss.seam.mock.MockExternalContext$AttributeMap.remove(MockExternalContext.java:471)
at org.jboss.seam.contexts.BasicContext.remove(BasicContext.java:73)
at org.jboss.seam.Component.newInstance(Component.java:2070)

=> need to cleanup the mock faces context in any case.