Improve exception messages
Improve exception messages

This patch improves some exception messages.

TrivialFix

Depends-On: I1b97fe8541c1c1325aaad739a30e5989c1fd806a
Change-Id: I71d107bed3685e4c7ac9a4fbae7172941f1e10e3

openstack/tap-as-a-service,stackforge/tap-as-a-service,stackforge/tap-as-a-service,openstack/tap-as-a-service