Remove VPN specific exception

Remove VPN specific exception

This exception is an overkill, and can be safely removed. The tests
affected were not designed to cover any regression as they were asserting
that the mocked Exception was being raised, defeating the very purpose of
catching the regression; they have been revised to ensure that the checks
are not misplaced in future revision of the code, or that they behave the
way they are supposed to.

Partially-Implements: blueprint services-split
Depends-On: https://review.openstack.org/#/c/153543/

Change-Id: I22cc8cd4383259dd4ee20bcbc041d589172d88c8