Add PerformInvite and refactor how errors get handled (#1158)

* Add PerformInvite and refactor how errors get handled

- Rename `JoinError` to `PerformError`
- Remove `error` from the API function signature entirely. This forces
  errors to be bundled into `PerformError` which makes it easier for callers
  to detect and handle errors. On network errors, HTTP clients will make a
  `PerformError`.

* Unbreak everything; thanks Go!

* Send back JSONResponse according to the PerformError

* Update federation invite code too

roomserver/api/input.go

@@ -76,21 +76,9 @@ type TransactionID struct {
 	TransactionID string `json:"id"`
 }
 
-// InputInviteEvent is a matrix invite event received over federation without
-// the usual context a matrix room event would have. We usually do not have
-// access to the events needed to check the event auth rules for the invite.
-type InputInviteEvent struct {
-	RoomVersion     gomatrixserverlib.RoomVersion             `json:"room_version"`
-	Event           gomatrixserverlib.HeaderedEvent           `json:"event"`
-	InviteRoomState []gomatrixserverlib.InviteV2StrippedState `json:"invite_room_state"`
-	SendAsServer    string                                    `json:"send_as_server"`
-	TransactionID   *TransactionID                            `json:"transaction_id"`
-}
-
 // InputRoomEventsRequest is a request to InputRoomEvents
 type InputRoomEventsRequest struct {
-	InputRoomEvents   []InputRoomEvent   `json:"input_room_events"`
-	InputInviteEvents []InputInviteEvent `json:"input_invite_events"`
+	InputRoomEvents []InputRoomEvent `json:"input_room_events"`
 }
 
 // InputRoomEventsResponse is a response to InputRoomEvents