[WIP] [TwigBundle] [Exception] Make return value similar to error.json.twig

| Q             | A
| ------------- | ---
| Bug fix?      | I guess this is a bug
| New feature?  | The exception structure changes for the default
| BC breaks?    | The exception structure changes for the default
| Deprecations? | [yes|no]
| Tests pass?   | [yes|no]
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

The result for `error.json.twig` and `exception.json.twig` differ making the client forced to check for it's result.

We think the structure should be the same to make the ie a javascript client try to respond similar for --env=dev|prod
It's going to be in 2.6 and this is a BC break, so you should prefix the line with `[BC BREAK]` (as such, I would also change `fixed` by `changed`.)