Remove filled class from errors ul when reseting

Calling reset removes the error `<li>`s, but leaves the empty `<ul>`.

![screen shot 2014-09-23 at 12 08 30 pm](https://cloud.githubusercontent.com/assets/310707/4375405/f9d7f704-433b-11e4-8592-dc777579554c.png)

This fixes https://github.com/guillaumepotier/Parsley.js/issues/626 and https://github.com/guillaumepotier/Parsley.js/issues/734