Improve error UX: give direction when the user failed to migrate

Copy of Issue 110:

At the moment the result of forgetting to migrate your database is this error:

JBWEB000065: HTTP Status 500 - org.jruby.exceptions.RaiseException: (DatabaseError) Java::OrgPostgresqlUtil::PSQLException: ERROR: relation "sequel_constraint_validations" does not exist

This is accurate as all get-out, but not exactly helpful. We should include a high level error filter and dolly that up with some hints like "you should run `razor-admin migrate-database` to fix this problem, because it will fix it right up; we didn't migrate ourselves automatically because ${reasons}".

...or just migrate automatically ourselves. (Also, we should check if migrations are current and do something reasonable in the situation that we don't match the on-disk migrations, like politely 500 everything with clear instructions.)