Don't show rectangular region editor if failed to take a screenshot

Summary:
For example, some Xorg security extension may forbid to take
screenshots. There is nothing to edit in this case, moreover
the region editor resizes to a null size thus leaving spectacle
in an abnormal state.

Reviewers: Spectacle, ngraham

Reviewed By: Spectacle, ngraham

Subscribers: ngraham

Tags: Spectacle

Differential Revision: https://phabricator.kde.org/D15425