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/pretty-format/build/plugins/Immutable.d.ts

@@ -0,0 +1,12 @@
+/**
+ * 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.
+ */
+import { Config, Printer, NewPlugin } from '../types';
+export declare const serialize: (val: any, config: Config, indentation: string, depth: number, refs: any[], printer: Printer) => string;
+export declare const test: (val: any) => boolean;
+declare const plugin: NewPlugin;
+export default plugin;
+//# sourceMappingURL=Immutable.d.ts.map