heat engine : avoid returning empty resource error strings

heat engine : avoid returning empty resource error strings

Avoid the possibility of returning an empty string (when Exceptions
have not got a string message), or parser.py will treat the error
as success and the resource operation failure will not be correctly
reflected in the resource state.

fixes bug 1130270

Change-Id: I1c55dea1f9615cd4b037802ff8c1066694bffea6