fromExternalizableId should just return null, not throw IAE, if the job has...

fromExternalizableId should just return null, not throw IAE, if the job has since been renamed/move/deleted.
(It could already return null if the run has since been deleted; #1264 failed to mark this properly.)
Reserving IAE for inputs that getExternalizableId could not actually have produced,
and making sure to also throw this in case of non-numeric text after the hash.
Adding Javadoc for both methods.