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/stubObject.js

@@ -0,0 +1,23 @@
+/**
+ * This method returns a new empty object.
+ *
+ * @static
+ * @memberOf _
+ * @since 4.13.0
+ * @category Util
+ * @returns {Object} Returns the new empty object.
+ * @example
+ *
+ * var objects = _.times(2, _.stubObject);
+ *
+ * console.log(objects);
+ * // => [{}, {}]
+ *
+ * console.log(objects[0] === objects[1]);
+ * // => false
+ */
+function stubObject() {
+  return {};
+}
+
+module.exports = stubObject;