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/_baseEach.js

@@ -0,0 +1,14 @@
+var baseForOwn = require('./_baseForOwn'),
+    createBaseEach = require('./_createBaseEach');
+
+/**
+ * The base implementation of `_.forEach` without support for iteratee shorthands.
+ *
+ * @private
+ * @param {Array|Object} collection The collection to iterate over.
+ * @param {Function} iteratee The function invoked per iteration.
+ * @returns {Array|Object} Returns `collection`.
+ */
+var baseEach = createBaseEach(baseForOwn);
+
+module.exports = baseEach;