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/babel-jest/build/index.d.ts

@@ -0,0 +1,16 @@
+/**
+ * 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 { Transformer } from '@jest/transform';
+import { TransformOptions } from '@babel/core';
+interface BabelJestTransformer extends Transformer {
+    canInstrument: true;
+}
+declare const transformer: BabelJestTransformer & {
+    createTransformer: (options?: TransformOptions) => BabelJestTransformer;
+};
+export = transformer;
+//# sourceMappingURL=index.d.ts.map