load returns 0 rather than erroring if not a skip file

src/skip.c

@@ -35,7 +35,8 @@ struct sw_skip *sw_skip_load(char *filename) {
 
 	fread(magic, 1, 4, fil);
 	if(strcmp("SWZP", magic) != 0) {
-		sw_error("not a .skip file");
+		sw_log("not a .skip file");
+		return 0;
 	}
 
 	fread(&version, 1, 1, fil);