In castellan 4.4.0, we fixed the wrong exception MockKeyManager.get
raised in case the requested key does not exist. This change interferes
with the way one unit test case ensure a key is gone.
This replaces the asserted exception by generic Exception so that we
can unblock u-c bump now. We can later replace it by the specific and
correct exception (ManagedObjectNotFoundError) once castellan 4.4.0 is
pulled to u-c.
[1] 2cc410f56e7275d982bca95aa65cd11e22fc7c3c
Change-Id: I8cc1420e8b16ce0bc74314fd7b8aabf6e133abd8