Team.addPlayer() should return an error if used before the player has been configured

[KianBrose]

I recently suffered from an issue where for some reason, even though the player was present in the team's members, it was not present in the playerMembers. It turns out that this was caused due to me adding the player in a pre-configure event that caused the "getPlayers()"'s getOnlinePlayerByUsername to fail as the player was not logged in yet. This would have been very hard to figure out without a debugger. The suggestion as per matt's discord message is to add an error if it's used before the player is configured.