Previously, if the configured error controller responded with a
redirect to an error caused by an exception, standalone Tomcat would
render its default error page for the original exception. This
occurred because ErrorPageFilter sets the
javax.servlet.error.exception request attribute prior to dispatching
to the error controller and then does not clear it. As the request
unwinds, Tomcat's ErrorReportValve notices that the attribute is set
and renders an error page for the exception that is the attribute's
value.

This commit updates ErrorPageFilter to remove the
javax.servlet.error.exception and javax.servlet.error.exception_type
attributes upon successful completion of a forward to the error
controller. This prevents Tomcat from rendering an error page for
an exception that has already been handled by the error controller.

Closes gh-7920