extensionDownloader: Use registered D-Bus error

Instead of returning ad-hoc errors, register a custom error domain
and return appropriate GLib.Errors.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3159>

js/misc/dbusErrors.js

@@ -41,3 +41,13 @@ const notificationErrorNames = [
 export const NotificationErrors =
     registerErrorDomain('Notifications', notificationErrorNames, 'org.gtk');
 export const NotificationError = createErrorEnum(notificationErrorNames);
+
+const extensionErrorNames = [
+    'InfoDownloadFailed',
+    'DownloadFailed',
+    'ExtractFailed',
+    'EnableFailed',
+];
+export const ExtensionErrors =
+    registerErrorDomain('Extensions', extensionErrorNames);
+export const ExtensionError = createErrorEnum(extensionErrorNames);