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/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(