Fix formatting of heat exceptions

Heat now consistently formats error messages in returned HTTP json
and heatclient HTTPException makes every attempt to capture these
messages.

This means that the safest way to display an error or validation
message from Heat is to rely on the existing behaviour of
Horizon's exceptions.handle.

Change-Id: I80cdb67df87cb5192f220bc78a156d6e4113112f
Closes-Bug: #1241395
(cherry picked from commit 2c7db077b265c25573bfef73586a539fa639f65f)