Job reservation: Use standalone exception handler

[andrewberls]

Previously the job reservation function used the standard exception handler, which logs errors to timbre and sends them to Sentry. This led to lots of ineligible status exceptions in Sentry, which is a non-actionable item that should be suppressed. The approach here is changed to use a separate exception handler for job reservation, which catches/logs ineligible starts and logs them before bailing out (no Sentry).

By doing this, only 1 remaining caller of the default handler remained, so we can just inline it with the standard job exception handler.

[elliot42]

:+1: I think there's some slight code golf possible on reserve-exception-handler but not a big deal

[elliot42]

:+1:

[elliot42]

Squishes?