Add more information to generic _add_floating_ip exception message

When we fail in some unexpected way in the _add_floating_ip method it'd
be good to have more information in the error message that's passed back
to the caller so they don't have to dig into the logs for the variables
involved, especially since the fixed IP might not be provided on the
request.

We're seeing messaging timeouts in the gate and it'd be nice to have the
error information right up front so we can fingerprint on it in
elastic-recheck.

Related-Bug: #1349590

Change-Id: I7c19c5c1bc5d41a922435d78aca0160c45130276
Merge "Add more information to generic _add_floating_ip exception message"