When parsing exceptions from openstacksdk, exc.details is
desired to contain error message, but sometimes error may
exist in exc.message. Openstacksdk's HttpException has __str__
method to handle error message, so in senlinclient HttpException
object can show the right error message.
Change-Id: I9246ed3d47151ebb8e03cd13134d58749cd35785
Closes-bug: #1541217