Parse error object (in json format) returned by heat-api

With this fix, heatclient will display a clear error message when
encounter an server-side error. Additionally, the corresponding
traceback will be displayed when "--verbose" is set.

Change-Id: I99b828465f61478a3c63fcf549d044a62523be1f
Merge "Parse error object (in json format) returned by heat-api"