Don't show an error message when user cancels screenshot in progress on Wayland
Currently, Spectacle shows an error message whenever something goes wrong during the screenshot process:
On Wayland, if the user cancels a screenshot during the select window/region prompt, it also counts as an error (
org.kde.KWin.ScreenShot2.Error.Cancelled) and a message is displayed, even though there is no bug.
With this change, Spectacle no longer shows an error message for user cancellation.