patman: make run results better visible

patman: make run results better visible

For an occasional user of patman some failures are not obvious: for
instance when checkpatch reports warnings, the dry run still reports
that the email would be sent. If it is not dry run, the warnings are
shown on the screen, but it is not clear that the email was not sent.

Add some code to report failure to send email explicitly.

Tested by running the script on a patch with style violations,
observed error messages in the script output.

Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>