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/api.go

@@ -18,11 +18,17 @@ type RoomserverInternalAPI interface {
 		response *InputRoomEventsResponse,
 	) error
 
+	PerformInvite(
+		ctx context.Context,
+		req *PerformInviteRequest,
+		res *PerformInviteResponse,
+	)
+
 	PerformJoin(
 		ctx context.Context,
 		req *PerformJoinRequest,
 		res *PerformJoinResponse,
-	) error
+	)
 
 	PerformLeave(
 		ctx context.Context,