Starting/Stopping actions should report if something goes wrong

how to reproduce:

1) connect to a cloud
2) open the instances subtree of the connection
3) select an instance
4) stop a stopped instance

Actual result:
nothing happens, no error is reported

Expected result:
The UI should report that there was an error performing this action since the instance is already stopped.

This usecase is a synthetic one since the Stop-action should not be available for instances that are stopped. It is exemplary for the general behaviour where no errors in actions get reported though.