mqarender: print error message if filenames not given

mqarender.c

@@ -85,8 +85,10 @@ int main(int argc, char **argv)
 	argc -= optind;
 	argv += optind;
 
-	if (argc < 2)
+	if (argc < 2) {
+		fprintf(stderr, "Input and output filenames must be given\n");
 		return 1;
+	}
 
 	memset(&infmt, 0, sizeof(infmt));
 	infile = sf_open(argv[0], SFM_READ, &infmt);