Properly encode heat.common.exception in rpc

Several types of exceptions can be raised inside heat.engine.service.
This patch ignores those which are in the heat.common.exception.py
file and encodes them into user errors.  Prior to this patch, only
some exceptions were properly encoded.  This patch assumes all
exceptions in heat.common.exception should be encoded in this way
in line with how oslo.messaging allows exmods to be specified.

Closes-Bug: 1287445

Change-Id: Ifb1901fbc32d574439e648643f49ae509d139aa7
Merge "Properly encode heat.common.exception in rpc"