ShowExceptions renders original exceptions

Current ShowExceptions middleware behavior is to use the wrapping exception to set the response. So all template errors are shown as 500's even when a 404 would be indicated from the wrapped exception.

This patch finds wrapped exceptions which have a response known to ShowExceptions and use that instead. If the wrapped exception is not known to ShowExceptions, the wrapping exception is used instead.

The original exception (i.e. the wrapping exception) continues to be logged.

Les Hill
Sandro Turriate