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/constant.js

@@ -0,0 +1,26 @@
+/**
+ * Creates a function that returns `value`.
+ *
+ * @static
+ * @memberOf _
+ * @since 2.4.0
+ * @category Util
+ * @param {*} value The value to return from the new function.
+ * @returns {Function} Returns the new constant function.
+ * @example
+ *
+ * var objects = _.times(2, _.constant({ 'a': 1 }));
+ *
+ * console.log(objects);
+ * // => [{ 'a': 1 }, { 'a': 1 }]
+ *
+ * console.log(objects[0] === objects[1]);
+ * // => true
+ */
+function constant(value) {
+  return function() {
+    return value;
+  };
+}
+
+module.exports = constant;