better exceptions for testing.html

Now, exceptions are throw properly as angry red text with a stack trace rather than black text with a stack trace whose links don't always work.

## Before
![screen shot 2014-07-14 at 4 30 07 pm](https://cloud.githubusercontent.com/assets/3344958/3578747/51e897c8-0bb0-11e4-96e8-2daefa8ce549.png)
## After
![screen shot 2014-07-14 at 4 28 08 pm](https://cloud.githubusercontent.com/assets/3344958/3578744/48ea8ab4-0bb0-11e4-9454-efc8da6b6fbf.png)

Also, errors don't stop all rendering: other tests after the failing one will continue to render.