[TwigBridge] Prevent code extension to display warning

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

During functional testing with phpunit and browserkit (and all their friends) when the application returns a `4XX` or a `5XX` response, symfony displays the trace with code context.

During a training, few people experienced a very weird issue with php 5.4, symfony 2.3.2, phpunit 3.7, Windows 7 or 8 and SensioLabsDesktop 0.5.

When they run functional  tests, and the application returns a `404`, in the "console" there was lot of warnings. I don't remember, sorry, but it was something about unexpected character and/or encoding. With the `@`, no more warnings.

I can't reproduce this issue on my computer (not windows). If needed, I can try to reproduce this bug on Monday at work.