If bug 0 is entered, don't bother hitting BMO. Immediately error back.

Summary: We check for number format but not 0. We should just kill this without hitting BMO.

Test Plan: Test with (1) adding a non-existant bug (should take a moment), (2) 0 (should error back immediately), and (3) a valid bug number

Reviewers: dkl, dkl_admin

Reviewed By: dkl

Subscribers: dkl_admin

Differential Revision: https://phabricator.services.mozilla.com/D98