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/_baseRest.js

@@ -0,0 +1,17 @@
+var identity = require('./identity'),
+    overRest = require('./_overRest'),
+    setToString = require('./_setToString');
+
+/**
+ * The base implementation of `_.rest` which doesn't validate or coerce arguments.
+ *
+ * @private
+ * @param {Function} func The function to apply a rest parameter to.
+ * @param {number} [start=func.length-1] The start position of the rest parameter.
+ * @returns {Function} Returns the new function.
+ */
+function baseRest(func, start) {
+  return setToString(overRest(func, start, identity), func + '');
+}
+
+module.exports = baseRest;