Add and use M_ROOM_IN_USE (#1972)

Signed-off-by: nivekuil <mail@nivekuil.com>

Co-authored-by: Neil Alexander <neilalexander@users.noreply.github.com>

clientapi/jsonerror/jsonerror.go

@@ -111,6 +111,12 @@ func UserInUse(msg string) *MatrixError {
 	return &MatrixError{"M_USER_IN_USE", msg}
 }
 
+// RoomInUse is an error returned when the client tries to make a room
+// that already exists
+func RoomInUse(msg string) *MatrixError {
+	return &MatrixError{"M_ROOM_IN_USE", msg}
+}
+
 // ASExclusive is an error returned when an application service tries to
 // register an username that is outside of its registered namespace, or if a
 // user attempts to register a username or room alias within an exclusive

clientapi/routing/createroom.go

@@ -325,7 +325,10 @@ func createRoom(
 			return jsonerror.InternalServerError()
 		}
 		if aliasResp.RoomID != "" {
-			return util.MessageResponse(400, "Alias already exists")
+			return util.JSONResponse{
+				Code: http.StatusBadRequest,
+				JSON: jsonerror.RoomInUse("Room ID already exists."),
+			}
 		}
 
 		aliasEvent = &fledglingEvent{
@@ -484,7 +487,10 @@ func createRoom(
 		}
 
 		if aliasResp.AliasExists {
-			return util.MessageResponse(400, "Alias already exists")
+			return util.JSONResponse{
+				Code: http.StatusBadRequest,
+				JSON: jsonerror.RoomInUse("Room alias already exists."),
+			}
 		}
 	}