[DX] Easier template customizing with a query parameter

Hi guys!

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #7446, #1486
| License       | MIT
| Doc PR        | TODO

This follows #7446. Here's a screenshot:

![screen shot 2014-07-05 at 2 38 34 pm](https://cloud.githubusercontent.com/assets/121003/3487411/fa8ec816-047b-11e4-886b-8ebe7959eefc.png)

When you click the link, voila! You see your real error template. It works by adding a `_real_template=1` query parameter, which the `ExceptionController` looks for.

The styling is a little rough on the link, as is the text, so I'm open to suggestions.

**TODOS**
* [ ] Open up a docs PR once we're settled on approach

Thanks!