style: Clean up whoami code

changelog.d/1276.bugfix

@@ -0,0 +1 @@
+Fixed the whoami endpoint returning HTTP 404 instead of HTTP 403, which confused some appservices. Contributed by @nex.

src/api/client/account.rs

@@ -724,7 +724,7 @@ pub(crate) async fn change_password_route(
 	Ok(change_password::v3::Response {})
 }
 
-/// # `GET _matrix/client/r0/account/whoami`
+/// # `GET /_matrix/client/v3/account/whoami`
 ///
 /// Get `user_id` of the sender user.
 ///
@@ -733,11 +733,17 @@ pub(crate) async fn whoami_route(
 	State(services): State<crate::State>,
 	body: Ruma<whoami::v3::Request>,
 ) -> Result<whoami::v3::Response> {
+	let is_guest = services
+		.users
+		.is_deactivated(body.sender_user())
+		.await
+		.map_err(|_| {
+			err!(Request(Forbidden("Application service has not registered this user.")))
+		})? && body.appservice_info.is_none();
 	Ok(whoami::v3::Response {
 		user_id: body.sender_user().to_owned(),
 		device_id: body.sender_device.clone(),
-		is_guest: services.users.is_deactivated(body.sender_user()).await?
-			&& body.appservice_info.is_none(),
+		is_guest,
 	})
 }