resolved: change error handling for manager_etc_hosts_read()

resolved: change error handling for manager_etc_hosts_read()

The choice what errors to ignore is left to the caller, and the caller is
changed to ignore all errors.

On error, previously read data is kept. So if e.g. an oom error happens, we
will continue to return slightly stale data instead of pretending we have no
entries for the given address. I think that's better, for example when
/etc/hosts contains some important overrides that external DNS should not be
queried for.