Specific errcode for when a user invited to a room is already present in the room

Nobody has claimed this yet.

Trying to invite a user into a room in which they are already present will give you a pretty generic `M_FORBIDDEN`. While this is perfectly valid, it does mean applications do not have a spec complaint way to treat this case differently to other errors. The only way to distinguish a failure due to membership conflict and a failure due to other reasons is by checking the `error` body for a "is already in the room" (in the case of Synapse).

I'd argue that there should be a specific error code for when a membership change was not possible, so applications (such as bridges) trying to ensure membership in a room can short circuit if the error returned tells them that the user is already joined.

Review the Matrix room-invite specification and its existing error-code conventions, focusing on how an invite for an already-present user is currently described. Check whether the proposed distinction needs protocol-wide agreement. Done means the specification clearly defines the applicable error response so clients can distinguish this membership conflict from other failures.

Written by the indexing model from the issue text.