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

This is a continuation of I32c83ef15fece35526739c4fcdfccf7a275c7033 as
another stub_out_client_exceptions() usage got merged by
I4120156db1499dfd3ed22095e528787eb73d33a6.

blueprint: oslo-messaging
Change-Id: If4ff9dba58e9f94fab80be804a98c76fff3267b7
Merge "Use ExceptionHelper to bypass @client_exceptions"