!adduser, !addchan: Changed language in error messages.

Signed-off-by: Martin Stone <martin@d7415.co.uk>
(cherry picked from commit aced3230d1a089fc56ada42cba9ebcdbf8213857)
!channels, !editchan, !showchan: Adapted from acl branch.

Signed-off-by: Martin Stone <martin@d7415.co.uk>
(cherry picked from commit 2d8ff08d92398458091bad5e8d266bb0be2d45ab)