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/expect/build/fakeChalk.js

@@ -0,0 +1,28 @@
+'use strict';
+
+var _ansiStyles = _interopRequireDefault(require('ansi-styles'));
+
+function _interopRequireDefault(obj) {
+  return obj && obj.__esModule ? obj : {default: obj};
+}
+
+/**
+ * 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.
+ */
+const returnInput = str => str;
+
+const allColorsAsFunc = Object.keys(_ansiStyles.default)
+  .map(style => ({
+    [style]: returnInput
+  }))
+  .reduce((acc, cur) => Object.assign(acc, cur));
+Object.keys(allColorsAsFunc)
+  .map(color => allColorsAsFunc[color])
+  .forEach(style => {
+    Object.assign(style, allColorsAsFunc);
+    Object.assign(returnInput, style);
+  });
+module.exports = allColorsAsFunc;