add util.errorWithNode method that returns an error with mapped location properties

lib/6to5/util.js

@@ -36,6 +36,13 @@ exports.parse = function (code, opts) {
   }
 };
 
+exports.errorWithNode = function (node, msg) {
+  var err = new Error(msg);
+  err.lineNumber = node.loc.start.line;
+  err.column = node.loc.start.column;
+  return err;
+};
+
 exports.canCompile = function (filename) {
   return path.extname(filename) === ".js";
 };