Don't translate StatusPacket to os.Err(NotExist|Permission)

[greatroar]

Doing this loses potentially useful information for the client. They should instead check with errors.Is(err, fs.ErrNotExist), etc.; the StatusPacket type has an Is method for this already.

[puellanivis]

Hm… 🤔 I can’t see any reason against this. It’s also for the dev-v2 branch, and so the potential change of the behavior would be isolated to a future breaking change.

And as noted, unlike io.EOF that gets a lot of regular == io.EOF checks in the standard library and elsewhere, these errors really should be using errors.Is(err, fs.ErrNotExist) and errors.Is(err, fs.ErrPermission) as called out in the os.IsNotExist and os.IsPermission.

The openssh-portable sftp-server does not return any more useful information in the error message for these, but other servers might (like ours).

[greatroar]

IIRC, the translation to os.ErrNotExist was actually a breaking change introduced in a minor revision, before errors.Is was a thing.

[puellanivis]

It seems was introduced before this package even released as v1.0.0.

Regardless, we have the better option now, and v2 should support the new way rather than keep an old way.