action packages creates misleading ERROR log messages

Created by: JoKeyser

The action call actions/packages is-package-manager-busy directly returns error codes that indicate failure, even though the semantics are simply that the package manager is busy. That creates confusing log messages of the following kind

plinth.actions ERROR    Error executing command - ['sudo', '-n', '/usr/share/plinth/actions/packages', 'is-package-manager-busy']

Instead, the script should return a string like true or false, and (implicitly) return error code 0 (unless it breaks).