Add keyword argument missing at some exc.HTTPError subclass

Some exc.HTTPError subclass needs keyword argument ``explanation``.
If there is no ``explanation=``, the message of the argument is
recognized as ``detail``.

Change-Id: Idfc07b2bd71da31e44212d3296921e18d2024a63
Closes-Bug: #1264424
Merge "Add keyword argument missing at some exc.HTTPError subclass"