Catch JSONDecodeError when attempting to initialize JIRAClient.

Review Request #10144 — Created Sept. 12, 2018 and updated

Review Board

If a bug tracker is configured incorrectly, JIRAClient won't be able to
initialize, and will unhelpfully throw a JSONDecodeError instead of
something more useful. The only place where this manifests is in the bug
infobox (since we otherwise are just creating a link to the bug). The
result of this was that crash e-mails were being sent to the admin
whenever the infobox was triggered (with no visible effect to the user).

Ran unit tests.

Checks run (2 succeeded)
flake8 passed.
JSHint passed.