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-config/build/constants.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.
+ */
+export declare const NODE_MODULES: string;
+export declare const DEFAULT_JS_PATTERN = "^.+\\.[jt]sx?$";
+export declare const DEFAULT_REPORTER_LABEL = "default";
+export declare const PACKAGE_JSON = "package.json";
+export declare const JEST_CONFIG = "jest.config.js";
+//# sourceMappingURL=constants.d.ts.map