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

federationapi/routing/invite.go

@@ -98,15 +98,15 @@ func Invite(
 	)
 
 	// Add the invite event to the roomserver.
-	if err = api.SendInvite(
+	if perr := api.SendInvite(
 		httpReq.Context(), rsAPI,
 		signedEvent.Headered(inviteReq.RoomVersion()),
 		inviteReq.InviteRoomState(),
 		event.Origin(),
 		nil,
-	); err != nil {
+	); perr != nil {
 		util.GetLogger(httpReq.Context()).WithError(err).Error("producer.SendInvite failed")
-		return jsonerror.InternalServerError()
+		return perr.JSONResponse()
 	}
 
 	// Return the signed event to the originating server, it should then tell

federationapi/routing/send_test.go

@@ -97,12 +97,18 @@ func (t *testRoomserverAPI) InputRoomEvents(
 	return nil
 }
 
+func (t *testRoomserverAPI) PerformInvite(
+	ctx context.Context,
+	req *api.PerformInviteRequest,
+	res *api.PerformInviteResponse,
+) {
+}
+
 func (t *testRoomserverAPI) PerformJoin(
 	ctx context.Context,
 	req *api.PerformJoinRequest,
 	res *api.PerformJoinResponse,
-) error {
-	return nil
+) {
 }
 
 func (t *testRoomserverAPI) PerformLeave(