Update from error styles based on feedback

Dokku: _pending_

This pull request styles the error messages to distinguish them from normal links by giving them a coloured background. I initially used a light red as we had previously but this clashed with the red input background.

I also removed the placeholder text in the auth forms, the current text was placeholder text, but in reality these fields are familiar enough that no placeholder should be necessary.

![screen shot 2014-07-31 at 17 15 02](https://cloud.githubusercontent.com/assets/47144/3766498/79d4f5e8-18c5-11e4-9101-d6cf1ef69218.png)