Raise self-formatted exceptions in SQlite.

 - A helper, marconi.tests.util.Expected, is added to replace
   testtools.ExpectedException, since the later one does not obey
   the exception hierarchy.

 - ClaimDoesNotExist is added, similar to MessageDoesNotExist.

Change-Id: I8f592184cbf7f3bdab95e9adef1306055097cb6f