[Form] Fixed regression introduced when merging 2.1 into master
merged branch bschussek/bugfix (PR #6630)

This PR was merged into the master branch.

Commits
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c1aff96 [Form] Fixed regression introduced when merging 2.1 into master

Discussion
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[Form] Fixed regression introduced when merging 2.1 into master

Bug fix: yes
Feature addition: no
Backwards compatibility break: no
Symfony2 tests pass: yes
Fixes the following tickets: -
Todo: -
License of the code: MIT
Documentation PR: -
Enhanced the triggering of E_USER_DEPRECATED errors

- 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.
- Enhanced the deprecation messages to mention the replacement whenever
  possible.