Return full error message for HTTP errors

Non-HeatException, non-remote errors were getting their messages
truncated at the first newline and leaving incomplete error messages the
in json response.  We can only be certain that remote errors raised
through oslo.messaging will conform to the "message + \n + stack_trace"
formula

Closes-Bug: #1358417
Change-Id: I785c6e41101960ea7ceb00c0733c49c2a8aa909f