Exception handling should not use exc.message

In looking at
https://review.openstack.org/#/c/77613/3/openstack_auth/backend.py , it
occurred to me that many other instances of e.message are likely lurking
throughout our code. And especially because Alex Gaynor decided to weigh
in, I think we have a somewhat authoritative view of how this should be
solved.

Change-Id: If00dd70de07ffd278d72c7d5cdf9b0021da4663a
Closes-Bug: 1319918
Merge "Exception handling should not use exc.message"