fix: Adjust error codes to comply with MSC4190

src/api/client/account/register.rs

@@ -104,6 +104,14 @@ pub(crate) async fn register_route(
 	};
 
 	let (token, device) = if !body.inhibit_login {
+		// If UIAA is disabled, we can't create a device. In that case only appservices
+		// can reach this point in the first place, so we return an error for them.
+		if !services.config.oauth.compatibility_mode.uiaa_available() {
+			return Err!(Request(AppserviceLoginUnsupported(
+				"User-interactive appservice registration is not available on this server."
+			)));
+		}
+
 		// Generate new device id if the user didn't specify one
 		let device_id = body
 			.device_id