anyError helper method

I wrote helper method to use with MULTI(). It just returns first error, `null` if there is no any.
Usage example:
```js
flow.exec(function() {
  smth(MULTI());
  smth2(MULTI());
}, function(results) {
  var err;
  err = flow.anyError(results);
  if (err) {
    return this(err);
  }
  // ....
});
```