Abstract loading job file from disk

Move the work from _LoadJobUnlocked to _LoadJobFileFromDisk, which can
then be used in other contexts as well. Also, if we fail to deserialize
the job, archive it as well (before we archived it only if we failed to
create the related object, but kept it there if deserialization failed.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>