Tweaked the handling of error message in the login page

The template now receives the AuthenticationException instead of only
its message key, allowing to support translation parameters.
Other exceptions are not rendered anymore (it should not happen anyway
as the Security system always use an AuthenticationException to fill the
attribute).
Removed useless translations

The authentication exception messages are not translated using the
translations shipped with Symfony itself.
Switched to PSR-4 instead of using target-dir in composer
Bumped the Symfony dependency to 2.3