Display error messages inline, if appropriate.

r? @ngokevin @spasovski

## Before

![screen shot 2014-08-04 at 4 16 44 pm](https://cloud.githubusercontent.com/assets/23885/3804355/21a11b32-1c25-11e4-994f-1e6a76e5a218.png)

## After

![screen shot 2014-08-04 at 4 16 28 pm](https://cloud.githubusercontent.com/assets/23885/3804357/24dd0130-1c25-11e4-892d-3b7d5576390c.png)
This is probably ultimately better, since it's consistent with how DRF reports errors.