tweaked the deprecated error handler to actually display the deprecation message

I've also removed uninterested information (like the trigger_error stack
frame).
[HttpKernel] refactored the HTTP content renderer to make it easier to extend
merged branch fabpot/content-renderer-refactoring (PR #6810)

This PR was merged into the master branch.

Commits
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aadefd3 [HttpKernel] refactored the HTTP content renderer to make it easier to extend

Discussion
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[HttpKernel] refactored the HTTP content renderer to make it easier to extend

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | kinda
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

This makes the StreamedResponse logic reusable for other strategies and it also makes the RenderingStrategy interface less fuzzy about its contract.

That also makes features like #4470 easier to implement from the outside.

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by stof at 2013-01-20T11:01:29Z

:+1: