Fix job error message after unclean master shutdown

Fix job error message after unclean master shutdown

According to commit 599ee321eb, any job-related error messages should
be encoded within a Ganeti-specific error and not passed on as a
string, to allow for easier parsing.

For jobs suffering from an undesirable status after an unclean master
daemon shutdown, the message was not encoded, as reported in issue 618.
This patch fixes the problem.

Signed-off-by: Hrvoje Ribicic <riba@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>