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
c8d91b2c30