git-p4: add p4-pre-submit exit text

git-p4: add p4-pre-submit exit text

When the p4-pre-submit exits with a non-zero exit code, the application
will abort the process with no additional information presented to the
user. This can be confusing for new users as it may not be clear that
the p4-pre-submit action caused the error.

Add text to explain why the program aborted the submit action.

Signed-off-by: Ben Keene <seraphire@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>