Bug: The hostlist api will make an internal error if you try to update custom fields for nonexistent hosts

[oyvindhagberg]

The SQL statements here will fail if no matching row is found in hostinfo:
https://github.com/usit-gd/nivlheim/blob/master/server/service/api_hostlist_post.go#L151

• Consider running each host in a separate transaction instead of running the entire thing in one transaction. Trying to update a host that doesn't exist shouldn't prevent the other hosts in the list from being updated.
• When updating multiple hosts, if some of them don't exist, the API should give a notification but otherwise return a 2xx status code.