Allow session to return an error response object

Typically we want to have exceptions thrown when dealing with requests
that return an HTTP error. However when looking at integrating the
session object with other clients it becomes apparent that the exception
handling is sufficiently different that it is best for now to let the
existing error handling work.

Add an option to return the failed request rather than raise an
exception so existing clients can do there own error handling.

Blueprint: session-propagation
DocImpact: New session parameter.
Change-Id: I63ea034e7c6eaaf42d4329526a902677a8dd709d
Merge "Allow session to return an error response object"
Test commit