[FrameworkBundle] adds log for sub requests rendering with ignore_errors

When rendering a sub request with `ignore_errors` set to `true` (default behavior in non-debug mode), if sub request throws an exception, alternative content is rendered (if any) and nothing is logged (because the `kernel.exception` event is never dispatched). This PR just dispatches this event.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #6400