#585 Improve signup errors

I’ve made the experience for getting errors on signup much better. The panel is much more noticeable now, thanks to a bold header and alert symbol. The overall design has been cleaned up too.

Here’s what the original looks like:

![old-validation](https://cloud.githubusercontent.com/assets/5074763/3212635/7508a130-ef68-11e3-93f9-a4039b0edd69.png)


And here’s mine:

![new-validation](https://cloud.githubusercontent.com/assets/5074763/3212632/35640f56-ef68-11e3-85e2-adcdd227ba28.png)