propate no-match found nil file after find* channel based queries

[odeke-em]

Fixes #536.
Updates #528.

It is an offset of PR #528, caused by the new pattern remote.FindByPathM -- multiple which returns a channel. Originally a call

         files, err := req.Do()

        if err != nil {
                if err.Error() == ErrGoogleApiInvalidQueryHardCoded.Error() { // Send the user back the query information
                        err = fmt.Errorf("err: %v query: `%s`", err, expr)
                }
                return nil, err
        }

        if files == nil || len(files.Items) < 1 {
                return nil, ErrPathNotExists
        }

        first := files.Items[0]
        if len(p) == 1 {
                return NewRemoteFile(first), nil
        }
        return r.findByPathRecvRaw(first.Id, p[1:], trashed)

The original pattern guaranteed the reporting of unmatched files

and yet now

for rem := range remotesChan {
   ....
}

// Will never be run if the file didn't exist upstream.
yet at no point was a lack of the first match ever propagated back with the empty file