Properly handle message post with an empty iterable.

Storage drivers expect at least one item in messages iterable,
so I modified the WSGI driver to check when this is not
the case and return an error response to the client rather
than blindly passing the messages generator into the
driver's post method.

Fixes: bug #1169838

Change-Id: Ia2e512138a465af70110d88e8e11ae58ee5e4aab