Added checks to make sure the specified track exists

Added precondition checks to validate input files and paths to return more informative error messages as per #2

node_modules/jest-jasmine2/build/isError.d.ts

@@ -0,0 +1,11 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+export default function isError(potentialError: any): {
+    isError: boolean;
+    message: string | null;
+};
+//# sourceMappingURL=isError.d.ts.map