ReST API: Don't overwrite webob error messages

We don't want to replace the entire body of the error messages returned,
just add our detailed description at the end.

Change-Id: I5899854d1ed1766285a5cae2f14c24025ed92e4b
Signed-off-by: Zane Bitter <zbitter@redhat.com>
Merge "ReST API: Don't overwrite webob error messages"