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/_castRest.js

@@ -0,0 +1,14 @@
+var baseRest = require('./_baseRest');
+
+/**
+ * A `baseRest` alias which can be replaced with `identity` by module
+ * replacement plugins.
+ *
+ * @private
+ * @type {Function}
+ * @param {Function} func The function to apply a rest parameter to.
+ * @returns {Function} Returns the new function.
+ */
+var castRest = baseRest;
+
+module.exports = castRest;