Don't show error message if sharing is cancelled by the user

Summary: Cancelling a share is usually intentional and should not result in a scary looking error message

Test Plan: Cancel a share from Dolphin, don't get an error message any more

Reviewers: apol, ngraham

Reviewed By: apol, ngraham

Subscribers: broulik, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D21989