Most notably this happens after blank database is initialized with --initdb,
but can happen also in other conditions.

The error checking changes modified behaviour if the file does not exist.
Explicitly check for ENOENT and ignore it. But the behaviour is improved
from earlier as now e.g. EPERM will be detected and reported correctly.

Fixes #10679

Fixes: 6da3e8eb "istream, archive, db: convert db and tar function to use istream"