Add HIDE_RESTRICTED_ACCESS option to supress access errors.

Use-case is restricted-access bots in populated channels, so that bot A
may ignore "!cmd" message from user M (intended for another bot or user)
without polluting channel with error messages.

Might also be seen as a security feature, masquerading bot presence, hiding precise error messages and not revealing which (potentially exploitable) commands the bot has (e.g. "!asd" will say "access denied" or be ignored instead of "no command !asd"), but admittedly, that's not the point of the change.