Fix gnt-job list argument handling

Currently QueryJob returns "None" when a wrong job ID is passed.
Handle this in gnt-job list, by printing an error for each wrong job,
and still giving output for all the jobs which actually do exist.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>