[Debug] cleanup interfaces before 2.5-final

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | none
| License       | MIT
| Doc PR        | none

This PR is targeted at cleaning up interfaces before 2.5 final:
- ExceptionHandlerInterface has never been released in a stable Symfony, lets drop it, not deprecate it,
- generalize a little bit how fatal errors are handled and make them take the same path as uncaught exceptions.