[ticket/11116] Made ACP "extension not available" error message E_USER_WARNING

The error message generated when an administrator tries to enable an
incompatible extension used to be displayed in the green of the default
E_USER_NOTICE. Changed it to E_USER_WARNING to make it appear in red, as
requested in the ticket.

PHPBB3-11116
Merge remote-tracking branch 'rechosen/ticket/11116' into develop

* rechosen/ticket/11116:
  [ticket/11116] Made ACP "extension not available" error message E_USER_WARNING