[2.3][TwigBundle] Add plain-text exception to exception page

Bug fix: no
Feature addition: yes
Backwards compatibility break: no
Symfony2 tests pass: yes
License of the code: MIT

This is very useful for support. Today I asked somebody for a stack trace on IRC. I don't want screenshots, I want text. And there is currently no easy way to get it.

This patch adds a text-based exception message to the exception page. The box is at the very bottom of the page and it is collapsed by default.

Screen shot:

![Screen Shot 2013-01-08 at 4 04 45 AM](https://f.cloud.github.com/assets/88061/49092/315c5b76-5940-11e2-8865-cf4e86fbfd1e.png)