Don't run server if database migrations fail

[matthew-white]

Right now if database migrations fail, start-odk.sh will still run the server. However, I think we want start-odk.sh to end immediately if database migrations fail and to not run the server. Otherwise you would have a server whose code doesn't match the database. In that case, many requests would result in a database error, and even if a request seems to succeed, it'd be harder to reason about the resulting state of the database.

For similar reasons, I think we also shouldn't run cron if migrations fail (also in start-odk.sh).