[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        | -

This is a first proposal for enabling error handling (mainly logging) in prod envs.
I'd like to make it for 2.6 but need help to make the best decisions.
@Koc worked on the subject but issue #8281 remains.

This misses configuration options for the ErrorHandler and for php ini settings related to error handling:
I'd like to allow apps to define any log channel per log level,
and I'd also like to be able to `ini_set('error_log', ...)` through symfony configuration system, with `app/logs/php.log` as default.

WIP state, but early feedback welcomed. We have only a few days to make it.

https://github.com/symfony/symfony/pull/12062/files?w=1