In some scenarios, the ErrorPageFilter will want to forward the request
to an error page but the response has already been committed. One common
cause of this is when the filter’s running on WAS. WAS calls
flushBuffer() (which commits the response), upon a clean exit from a
servlet’s service method.

Previously, the filter would attempt the forward, even if the response
was committed. This would result in an IllegalStateException and a
possibly incomplete response that may also have an incorrect status
code.

This commit updates the ErrorPageFilter to check to see if the response
has already been committed before it attempts to forward the request to
the error page. If the response has already been committed, the filter
logs an error and allows the container’s normal handling to kick in.
This prevents an IllegalStateException from being thrown.

This commit also updates the response wrapper to keep track of when
sendError has been called. Now, when flushBuffer is called, if
sendError has been called, the wrapper calls sendError on the wrapped
response. This prevents the wrapper from suppressing an error when the
response is committed before the request handling returns to the error
page filter.

Closes gh-1575