Channel modes +k & +l: Always report an error when a parameter is missing

This relates to #290 and considerations which errors to show when: and I
think it is the better approach to give feedback instead of silently
failing.

Note that this code path is also used when handling modes of channels
defined in "[Channel]" blocks in configuration files: in this case the
client is the local server and we can't send messages to it, because it
has no socket connection! Therefore we need those "is_machine" checks
and log an error im this case.