Add error signal to KAbstractFileItemActionPlugin

This will allow the plugins to display errors to the user. The suggested docstring to use theparentWidget to handle errors can consequently be removed.

Task: https://phabricator.kde.org/T12164

The other described usecase in the task is currently not possible, because the job is started async and the plugin instance is deleted before it finishes. I am not sure how to best solve this, IMHO it could make sense to keep the KFileItemActions instances around longer and reuse the plugin instances.

Another thought: This could allow us to get rid of the QWidgets dependency, would that make sense for KF6?