Enhanced the triggering of E_USER_DEPRECATED errors

Bug fix: no
Feature addition: no
Backwards compatibility break: no
Deprecations: no
Symfony2 tests pass: yes
Fixes the following tickets: none

- Removed useless error handlers around FormEvent as the triggering has
  been fixed in it.
- Enhanced the triggering of deprecation errors for places where the BC
  method provide some user logic needing to be converted to a new way.
  For instance, AbstractType should trigger the error when the type extending it overwrites the deprecated methods instead of ``setDefaultOptions``, which was not the case previously.
- Enhanced the deprecation messages to mention the replacement whenever
  possible.