[BUGFIX] EXT:form - Fix utf-8 encoding for error message

This patchset fixes a problem with the error message. As soon as the
message contains utf-8 characters, they are not properly displayed.


Resolves: #71557
Releases: master
Change-Id: I18dfb0862e89680f69f35ae6437bad5f808c7569
Reviewed-on: https://review.typo3.org/44704


Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Reviewed-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Tested-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Tested-by: Daniel Goerz <ervaude@gmail.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>