Include backtrace in exceptions raised by hooks

This makes it easier to debug the underlying problem, which is usually
what you want to do.

Change-Id: Ic82c26f9833f6be4eca45dbf85d941d983d3d4db
Reviewed-on: http://gerrit.causes.com/44899
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>