[scripting] Make DBus invokable run method return result after execution

Summary:
In 403038 the user expected the DBus call to run to finish before
processing the next request. For a singleshot script that makes a lot of
sense, otherwise you have no idea when it finishes.

This also allows us to return errors.

CCBUG: 403038

Test Plan:
NUM=$(qdbus org.kde.KWin /Scripting org.kde.kwin.Scripting.loadScript /noFile.js Minimize)
qdbus org.kde.KWin /$NUM org.kde.kwin.Scripting.run
  Error: org.kde.kwin.Scripting.FileError
  Could not open /noFile.js

Running a real script behaved effectively the same as before.

Reviewers: #kwin, graesslin

Reviewed By: #kwin, graesslin

Subscribers: zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18127