WIP - 3.0 Improved error pages

The error pages in 3.0 are a bit bland and needed some sprucing up. This is still very much a work in progress, but I wanted to get some early feedback on the design and approach before I completed all the error templates. These changes do have some implications:

* Development errors use a special layout, with custom CSS and Javascript. This means they cannot be corrupted by app helpers and app layouts which should allow us to make error handling simpler in the future.
* Development errors don't have external stylesheets or js code. This is not a 'best practice' but allows the development errors to be self contained and not rely on external resources. Whoops takes a similar approach.

It ends up looking like:

![screen shot 2014-11-28 at 11 40 38 pm](https://cloud.githubusercontent.com/assets/24086/5233534/b8e7255c-7758-11e4-94b4-0777cdd939bf.png)

* The stack frames on the left are clickable and show the correct file excerpt
* The `toggle arguments` link shows the `Debugger::exportVar()` representations of the function arguments.

If the general approach and design are :+1: I'll finish off the remaining templates.
I'd also like to remove this "Error" word here. Some times it makes no sense when reading sentences that are developer hints instead of actual errors.