manager: remove manager_get_unit_from_dbus_path()
manager: remove manager_get_job_from_dbus_path()
core/test: rework function signatures of manager_load_unit() and manager_load_unit_prepare() to omit DBusError
core/test: rewrite function signature of manager_add_job* functions to omit DBusError
snapshot: rework snapshot_create() signature to omit DBusError