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/_flatRest.js

@@ -0,0 +1,16 @@
+var flatten = require('./flatten'),
+    overRest = require('./_overRest'),
+    setToString = require('./_setToString');
+
+/**
+ * A specialized version of `baseRest` which flattens the rest array.
+ *
+ * @private
+ * @param {Function} func The function to apply a rest parameter to.
+ * @returns {Function} Returns the new function.
+ */
+function flatRest(func) {
+  return setToString(overRest(func, undefined, flatten), func + '');
+}
+
+module.exports = flatRest;