[effects] Cleanup screenshot effect

Summary:
This change cleans up the screenshot effect a little bit.
* better check whether a screenshot is already being taken
* proper DBus error messages
* less duplication of error message strings
* don't keep the QDBusConnection around

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D3493