[SecurityBundle] error helper added symfony/symfony#11147
feature #11324 [SecurityBundle] error helper added symfony/symfony#11147 (i3or1s)

This PR was squashed before being merged into the 2.6-dev branch (closes #11324).

Discussion
----------

[SecurityBundle] error helper added symfony/symfony#11147

Added helper that extracts last authentication error and username.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | [ #11147 ]
| License       | MIT
| Doc PR        | symfony/symfony-docs#3996

Commits
-------

1722f60 [SecurityBundle] error helper added symfony/symfony#11147