Errors during job initialisation aren't logged as messages against the job

[GuySartorelli]

If a job fails to initialise, the error doesn't get added as a message against the job, which means you can't see from within the CMS admin UI what caused the failure.

As discussed in #374 (review), adding messages to the job currently requires a QueuedJob instance (which is what failed to initialise), despite the fact that the messages themselves are stored against a QueuedJobDescriptor record (which we do have at the time of the failure).