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!
Hmm, there is a problem implementing either of these, primarily because we don't have access to the original request object from within `ExceptionController`, so I can't determine if the original request was a GET or a POST.

I *do* have access to the original query parameters, but only by chance (because the query parameters are duplicated when duplicating the request).

@stof What's your opinion on passing some information (method at least, maybe also query parameters) to the `ExceptionController`? My preference would be just to only show the link if we have a GET request. Even then, I don't think we can catch all edge-cases (e.g. the user could be reading something from the DB or session, which is then removed, so refreshing doesn't give you the exact situation again).