[FrameworkBundle] enable ErrorHandler in prod
[Debug] make screaming configurable
[FrameworkBundle] register ErrorHandler at boot time
feature #12081 [FrameworkBundle] enable ErrorHandler in prod (nicolas-grekas)

This PR was merged into the 2.6-dev branch.

Discussion
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[FrameworkBundle] enable ErrorHandler in prod

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11053, #8281
| License       | MIT
| Doc PR        | -

-  a new debug.error_handler service is the registered PHP error handler, with ErrorHandler::register() as factory
- ErrorHandler::register() is patched so that it checks if the currently registered error handler is an instance of ErrorHandler - in which case it returns this instance and don't create a new one.
- DebugHandlersListener now listen to ConsoleEvents and re-injects fatal errors within the $app->renderException code path
- DebugHandlersListener also has a new $scream parameter to control is silenced errors are logged or not

Commits
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fac3cc4 [FrameworkBundle] register ErrorHandler at boot time
4acf5d3 [Debug] make screaming configurable
4d0ab7d [FrameworkBundle] enable ErrorHandler in prod