monitor: make hmp_handle_error return a boolean

monitor: make hmp_handle_error return a boolean

This turns the pattern

  if (err) {
     hmp_handle_error(mon, err);
     return;
  }

into

  if (hmp_handle_error(mon, err)) {
     return;
  }

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>