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>

git-p4.py

@@ -2405,7 +2405,15 @@ def run(self, args):
             sys.exit("number of commits (%d) must match number of shelved changelist (%d)" %
                      (len(commits), num_shelves))
 
-        if not run_git_hook("p4-pre-submit"):
+        try:
+            if not run_git_hook("p4-pre-submit"):
+                print("\nThe p4-pre-submit hook failed, aborting the submit.\n\nYou can skip " \
+                    "this pre-submission check by adding\nthe command line option '--no-verify', " \
+                    "however,\nthis will also skip the p4-changelist hook as well.")
+                sys.exit(1)
+        except Exception as e:
+            print("\nThe p4-pre-submit hook failed, aborting the submit.\n\nThe hook failed "\
+                "with the error '{0}'".format(e.message) )
             sys.exit(1)
 
         #