Fail prediction on unhandled error (#1981)
If the runner fails to handle an error this gets bubbled up to the
worker, which logs a warning and then merrily carries on. Errors
bubbling up in this way include things like failing to upload a file,
which should fail the prediction. Currently if a model fails to upload
one or more files for the prediction, the prediction will still be
marked as succeeded, but it won't have any files in the output.
