script:autobuild: Make sure we can send a failure mail

script:autobuild: Make sure we can send a failure mail

We should not run into an exception if the file doesn't exist.

Traceback (most recent call last):
  File "script/autobuild.py", line 1781, in <module>
    email_failure(-1, 'rebase', 'rebase', 'rebase',
  File "script/autobuild.py", line 1677, in email_failure
    f = open("%s/%s.stdout" % (gitroot, failed_tag), 'r')
FileNotFoundError: [Errno 2] No such file or directory:
  'samba-autobuild/rebase.stdout'

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Mon Jan 30 10:00:27 UTC 2023 on atb-devel-224