Fix a few job archival issues

This patch fixes two issues with job archival. First, the
LoadJobFromDisk can return 'None' for no-such-job, and we shouldn't add
None to the job list; we can't anyway, as this raises an exception:

  node1# gnt-job archive foo
  Unhandled protocol error while talking to the master daemon:
  Caught exception: cannot create weak reference to 'NoneType' object

After fixing this, job archival of missing jobs will just continue
silently, so we modify gnt-job archive to log jobs which were not
archived and to return exit code 1 for any missing jobs.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>