[ticket/11537] Adjust error message on ucp group manage to fit rest of ucp

The current look highly differed from the rest of the UCP. Additionally,
this caused the error message to be hardly noticeable at all.

PHPBB3-11537
Merge remote-tracking branch 'marc1706/ticket/11537' into develop-olympus

* marc1706/ticket/11537:
  [ticket/11537] Adjust error message on ucp group manage to fit rest of ucp