Improve handling of lost jobs

Currently, if the cli.JobExecutor class is being used, and one of the
jobs is being archived before it can check its result, it will raise a
stracktrace as _ChooseJob is not prepared to handle this case.

This case makes JobExecutor work better with lost jobs (it still reports
them as 'failed', but it doesn't break and returns a proper error
message), and modifies the generic FormatError to report the JobLost
exception properly, instead of as "Unhandled Ganeti Exception".

Since JobExecutor is hard to test properly, I only tested this manually,
via a fake invocation.

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