As described on http://projects.theforeman.org/issues/16081 - the fact_values helper is expecting that "fact_value.host" to return a meaningful value all the time.
However for certain hosts (DiscoveredHost now, but it could be others in the future) /fact_values shows a 500 because "fact_value.host" doesn't exist.

I think it'd be reasonable to expect fact_value.host to exist for Hosts in core, but we should show N/A if that cannot be found. This way the page will be more resilient to manual db manipulation, data corruption, or plain bugs like http://projects.theforeman.org/issues/16081 .