[Phanx]

Made some changes for joined/left guild messages, and to more easily support adding more message types:

local DESTINATION = UIErrorsFrame

local messageTypes = {
	{
		-- X has come online
		pattern = ERR_FRIEND_ONLINE_SS:gsub("%%s", "(.+)"):gsub("[%[%]]", "%%%1"),
		r = 0.3, g = 1, b = 0.3,
	},
	{
		-- X has gone online
		pattern = ERR_FRIEND_OFFLINE_S:gsub("%%s", "(.+)"),
		r = 0.6, g = 0.6, b = 0.6,
	},
	{
		-- X has joined the guild
		pattern = ERR_GUILD_JOIN_S:gsub("%%s", "(.+)"),
		r = 0.3, g = 1, b = 0.3,
	},
	{
		-- X has left the guild
		pattern = ERR_GUILD_LEAVE_S:gsub("%%s", "(.+)"),
		r = 0.6, g = 0.6, b = 0.6,
	},
}

local f = CreateFrame("Frame")
f:RegisterEvent("CHAT_MSG_SYSTEM")
f:SetScript("OnEvent", function(self, event, text)
	for i = 1, #messageTypes do
		local messageType = messageTypes[i]
		if text:match(messageType.pattern) then
			if not DESTINATION:IsMouseEnabled() then
				-- Remove link brackets if the destination frame doesn't support clicking
				text = text:gsub("%[(.-)%]", "%1")
			end
			return DESTINATION:AddMessage(text, messageType.r, messageType.g, messageType.b)
		end
	end
end)