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-worker/build/Farm.d.ts

@@ -0,0 +1,27 @@
+/**
+ * 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 { FarmOptions } from './types';
+export default class Farm {
+    private _computeWorkerKey;
+    private _cacheKeys;
+    private _callback;
+    private _last;
+    private _locks;
+    private _numOfWorkers;
+    private _offset;
+    private _queue;
+    constructor(numOfWorkers: number, callback: Function, computeWorkerKey?: FarmOptions['computeWorkerKey']);
+    doWork(method: string, ...args: Array<any>): Promise<unknown>;
+    private _getNextTask;
+    private _process;
+    private _enqueue;
+    private _push;
+    private _lock;
+    private _unlock;
+    private _isLocked;
+}
+//# sourceMappingURL=Farm.d.ts.map