screenshot: Return an error when operation is blocked

Right now _createScreenshot() returns a tuple that indicates failure
when a sender requests a screenshot operation before a previously
started operation finished.

However that doesn't work for the PickColor() method, as it uses a
different return type than the other methods.

Address this by returning an error instead, which works in any case;
arguably trying to start multiple operations in parallel is an error
by the caller more than it is a failed operation anyway.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1288

js/ui/screenshot.js

@@ -37,7 +37,9 @@ var ScreenshotService = class {
 
         let sender = invocation.get_sender();
         if (this._screenShooter.has(sender) || lockedDown) {
-            invocation.return_value(GLib.Variant.new('(bs)', [false, '']));
+            invocation.return_error_literal(
+                Gio.IOErrorEnum, Gio.IOErrorEnum.BUSY,
+                'There is an ongoing operation for this sender');
             return null;
         }