mv: Improve message when moving two or more files to non-directory.

The message text is from cp, which has had a nicer message for this since
2007 (PR bin/50656).

As with cp, the exit status changes from 64 to 1.

PR:		201083
MFC after:	1 week

bin/mv/mv.c

@@ -122,7 +122,7 @@ main(int argc, char *argv[])
 	 */
 	if (stat(argv[argc - 1], &sb) || !S_ISDIR(sb.st_mode)) {
 		if (argc > 2)
-			usage();
+			errx(1, "%s is not a directory", argv[argc - 1]);
 		exit(do_move(argv[0], argv[1]));
 	}