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/wrapper.go

@@ -98,16 +98,20 @@ func SendInvite(
 	rsAPI RoomserverInternalAPI, inviteEvent gomatrixserverlib.HeaderedEvent,
 	inviteRoomState []gomatrixserverlib.InviteV2StrippedState,
 	sendAsServer gomatrixserverlib.ServerName, txnID *TransactionID,
-) error {
-	request := InputRoomEventsRequest{
-		InputInviteEvents: []InputInviteEvent{{
-			Event:           inviteEvent,
-			InviteRoomState: inviteRoomState,
-			RoomVersion:     inviteEvent.RoomVersion,
-			SendAsServer:    string(sendAsServer),
-			TransactionID:   txnID,
-		}},
+) *PerformError {
+	request := PerformInviteRequest{
+		Event:           inviteEvent,
+		InviteRoomState: inviteRoomState,
+		RoomVersion:     inviteEvent.RoomVersion,
+		SendAsServer:    string(sendAsServer),
+		TransactionID:   txnID,
 	}
-	var response InputRoomEventsResponse
-	return rsAPI.InputRoomEvents(ctx, &request, &response)
+	var response PerformInviteResponse
+	rsAPI.PerformInvite(ctx, &request, &response)
+	// we need to do this because many places people will use `var err error` as the return
+	// arg and a nil interface != nil pointer to a concrete interface (in this case PerformError)
+	if response.Error != nil && response.Error.Msg != "" {
+		return response.Error
+	}
+	return nil
 }