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/_baseAssign.js

@@ -0,0 +1,17 @@
+var copyObject = require('./_copyObject'),
+    keys = require('./keys');
+
+/**
+ * The base implementation of `_.assign` without support for multiple sources
+ * or `customizer` functions.
+ *
+ * @private
+ * @param {Object} object The destination object.
+ * @param {Object} source The source object.
+ * @returns {Object} Returns `object`.
+ */
+function baseAssign(object, source) {
+  return object && copyObject(source, keys(source), object);
+}
+
+module.exports = baseAssign;