BUG/MINOR: server-state: Avoid warning on 'file not found'

BUG/MINOR: server-state: Avoid warning on 'file not found'

On a clean installation, users might want to use server-state-file and
the recommended zero-warning option. This caused a problem if
server-state-file was not found, as a warning was emited, causing
startup to fail.

This will allow users to specify nonexistent server-state-file at first,
and dump states to the file later.

Fixes #2190

CF: Technically speaking, this patch can be backported to all stable
    versions. But it is better to do so to 2.8 only for now.