Provide a better error message when the /dev entry is non-existant.

Due to the old message, I spent way more time debugging a diskless
root problem than it should have taken.

sbin/mount/mount_ufs.c

@@ -137,6 +137,9 @@ mount_ufs(argc, argv)
 				warnx("%s on %s: incorrect super block",
 					args.fspec, fs_name);
 			break;
+		case ENOENT:
+			warn("%s", args.fspec);
+			break;
 		default:
 			warn(NULL);
 			break;