[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 branch 'ticket/11116' of https://github.com/rechosen/phpbb3 into ticket/11116