Don't disguise CFN API exceptions

The exception disguise wrapper is for the native API which uses
the faultwrap filter to process the response. For HeatAPIException
subclasses (which are only raised by the CFN/CW API's) we need to
raise the unwrapped exception or the client sees a 500 response.

Change-Id: Iebfbc76d5cb26aa9ee61c5192d7fecc596aa83b9
Closes-Bug: #1291079