fix: add error message for failing zip extraction

Author: Bentroen

scripts/open_song_zip/open_song_zip.gml

@@ -6,7 +6,10 @@ function open_song_zip(filename) {
 		directory_delete_lib(dst_path);
 	}
 
-	zip_unzip(filename, dst_path);
+	var output = zip_unzip(filename, dst_path);
+	if (output < 0) {
+		throw("This doesn't appear to be a valid ZIP file!")
+	}
 
 	var song_path = dst_path + "song.nbs";
 	var sounds_path = dst_path + "sounds\\";