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/_baseForRight.js

@@ -0,0 +1,15 @@
+var createBaseFor = require('./_createBaseFor');
+
+/**
+ * This function is like `baseFor` except that it iterates over properties
+ * in the opposite order.
+ *
+ * @private
+ * @param {Object} object The object to iterate over.
+ * @param {Function} iteratee The function invoked per iteration.
+ * @param {Function} keysFunc The function to get the keys of `object`.
+ * @returns {Object} Returns `object`.
+ */
+var baseForRight = createBaseFor(true);
+
+module.exports = baseForRight;