cherry-pick/revert: Use error() for failure message

When cherry-pick fails after picking a large series of commits, it can
be hard to pick out the error message and advice.  Clarify the error
and prefix it with “error: ” to help.

Before:

	Automatic cherry-pick failed.  [...advice...]

After:

	error: could not apply 7ab78c9... Do something neat.
	[...advice...]

Noticed-by: Thomas Rast <trast@student.ethz.ch>
Encouraged-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/revert.c

@@ -524,8 +524,11 @@ static int do_pick_commit(void)
 	}
 
 	if (res) {
-		fprintf(stderr, "Automatic %s failed.%s\n",
-			mebuf.buf, help_msg());
+		error("could not %s %s... %s",
+		      action == REVERT ? "revert" : "apply",
+		      find_unique_abbrev(commit->object.sha1, DEFAULT_ABBREV),
+		      msg.subject);
+		fprintf(stderr, help_msg());
 		rerere(allow_rerere_auto);
 	} else {
 		if (!no_commit)