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/round.js

@@ -0,0 +1,26 @@
+var createRound = require('./_createRound');
+
+/**
+ * Computes `number` rounded to `precision`.
+ *
+ * @static
+ * @memberOf _
+ * @since 3.10.0
+ * @category Math
+ * @param {number} number The number to round.
+ * @param {number} [precision=0] The precision to round to.
+ * @returns {number} Returns the rounded number.
+ * @example
+ *
+ * _.round(4.006);
+ * // => 4
+ *
+ * _.round(4.006, 2);
+ * // => 4.01
+ *
+ * _.round(4060, -2);
+ * // => 4100
+ */
+var round = createRound('round');
+
+module.exports = round;