manager: do not ignore the return value from the main loop

manager: do not ignore the return value from the main loop

If manager_loop() fails, we would print an error message, but then actually
ignore the error in main(), and potentially execute the shutdown binary.
I'm not sure how likely this is to happen in practice, but it seems sloppy.
So let's do the cleanup, but actually freeze() if manager_loop() returned
an error.

invoke_main_loop() is refactored to return the manager objective. This way
we don't need to pass a separate parameter to specify whether we are
reexecuting. Subsequent patch will make further use of the returned objective.