show an error if logged in as a different user (bug 837201-yak)

not sure if this is the right way to go, r? @muffinresearch

![screenshot 2013-11-06 12 04 15](https://f.cloud.github.com/assets/74699/1486153/b4f61c84-471e-11e3-9323-6a29f3ae2a1c.png)