Added metrics pages for 4xx & 5xx errors

Added 3 pages...

First shows global 4xx and 5xx error rate.
Other two pages show 4xx per loadbalancer and 5xx per loadbalancer.

Its good that we are making more metrics pages, but the code is running a bit wild. I have tried to isolate these, and will put some effort into cleaning up the others too.

![errors](https://f.cloud.github.com/assets/76709/1508425/8e4c21a4-49f9-11e3-8a76-bcdadc06a2d3.png)