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/expect/build/extractExpectedAssertionsErrors.d.ts

@@ -0,0 +1,18 @@
+/**
+ * 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.
+ *
+ */
+declare const extractExpectedAssertionsErrors: () => ({
+    actual: any;
+    error: any;
+    expected: number;
+} | {
+    actual: string;
+    error: any;
+    expected: string;
+})[];
+export default extractExpectedAssertionsErrors;
+//# sourceMappingURL=extractExpectedAssertionsErrors.d.ts.map