Raise a specific exception (NotFound) if something is not found in the
database. Then we can match on this exception, rather than searching the
error message (which effectively makes the text of error messages part of
the API).
Change-Id: I22673ffb1407cf95b1fe24eaec8824039c1dda4e
Signed-off-by: Zane Bitter <zbitter@redhat.com>