Some p4 failures result in an error, but the info['code'] is not
set.  These include a bad p4 executable, or a core dump from p4,
and other odd internal errors where p4 fails to generate proper
marshaled output.

Make sure the info key exists before using it to avoid a python
traceback.
Signed-off-by: NPete Wyckoff <pw@padd.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>