`REDACT_FORBIDDEN` human-readable string is inaccurate

[slingamn]

<@jwheare> slingamn: when i try to redact my own message in a channel i get FAIL REDACT_FORBIDDEN: You are not authorized to delete because you are logged out

This message does not reflect the actual policy constraints here:

ergo/irc/histserv.go

Lines 103 to 110 in 2d44ab1

	// Returns:
	//
	// 1. `canDeleteAny` if the client allowed to delete other users' messages from the target, ie.:
	// - the client is a channel operator, or
	// - the client is an operator with "history" capability
	//
	// 2. `canDeleteSelf` if the client is allowed to delete their own messages from the target
	// 3. `canDeleteNone` otherwise

and should be clarified.