Use ExceptionHelper to bypass @client_exceptions

Rather than stubbing out catch_client_exception() to bypass the wrapping
of exceptions in ClientException, use the existing ExceptionHelper to do
it.

This is import for oslo.messaging, because there's no public API method
for us to stub out.

blueprint: oslo-messaging
Change-Id: I32c83ef15fece35526739c4fcdfccf7a275c7033
Merge "Use ExceptionHelper to bypass @client_exceptions"