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/lodash/isWeakMap.js

@@ -0,0 +1,28 @@
+var getTag = require('./_getTag'),
+    isObjectLike = require('./isObjectLike');
+
+/** `Object#toString` result references. */
+var weakMapTag = '[object WeakMap]';
+
+/**
+ * Checks if `value` is classified as a `WeakMap` object.
+ *
+ * @static
+ * @memberOf _
+ * @since 4.3.0
+ * @category Lang
+ * @param {*} value The value to check.
+ * @returns {boolean} Returns `true` if `value` is a weak map, else `false`.
+ * @example
+ *
+ * _.isWeakMap(new WeakMap);
+ * // => true
+ *
+ * _.isWeakMap(new Map);
+ * // => false
+ */
+function isWeakMap(value) {
+  return isObjectLike(value) && getTag(value) == weakMapTag;
+}
+
+module.exports = isWeakMap;