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-util/build/convertDescriptorToString.js

@@ -0,0 +1,39 @@
+'use strict';
+
+Object.defineProperty(exports, '__esModule', {
+  value: true
+});
+exports.default = convertDescriptorToString;
+
+/**
+ * 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.
+ */
+// See: https://github.com/facebook/jest/pull/5154
+function convertDescriptorToString(descriptor) {
+  if (
+    typeof descriptor === 'string' ||
+    typeof descriptor === 'number' ||
+    descriptor === undefined
+  ) {
+    return descriptor;
+  }
+
+  if (typeof descriptor !== 'function') {
+    throw new Error('describe expects a class, function, number, or string.');
+  }
+
+  if (descriptor.name !== undefined) {
+    return descriptor.name;
+  } // Fallback for old browsers, pardon Flow
+
+  const stringified = descriptor.toString();
+  const typeDescriptorMatch = stringified.match(/class|function/);
+  const indexOfNameSpace = // @ts-ignore: typeDescriptorMatch exists
+    typeDescriptorMatch.index + typeDescriptorMatch[0].length;
+  const indexOfNameAfterSpace = stringified.search(/\(|\{/);
+  const name = stringified.substring(indexOfNameSpace, indexOfNameAfterSpace);
+  return name.trim();
+}