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-watcher/build/JestHooks.d.ts

@@ -0,0 +1,17 @@
+/**
+ * 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 { JestHookSubscriber, JestHookEmitter } from './types';
+declare type AvailableHooks = 'onFileChange' | 'onTestRunComplete' | 'shouldRunTestSuite';
+declare class JestHooks {
+    private _listeners;
+    constructor();
+    isUsed(hook: AvailableHooks): number;
+    getSubscriber(): JestHookSubscriber;
+    getEmitter(): JestHookEmitter;
+}
+export default JestHooks;
+//# sourceMappingURL=JestHooks.d.ts.map