Warn when toggling erc-nicks-mode in target buffers

Warn when toggling erc-nicks-mode in target buffers

* lisp/erc/erc-nicks.el (erc-nicks-mode, erc-nicks-enable): This local
module isn't autoloaded like most global modules, in part to dissuade
users from attempting to enable it by running M-x erc-nicks-mode RET.
However, rather than signal an error upon detecting such an attempt,
ERC should explain that the only supported means of activation is via
`erc-modules'.  This change does that and then enables it anyway,
despite the degraded experience.