sftpfs: map path resolution error to permission denied

we do the same for os fs so that the problematic directory is excluded
from the webdav listing instead of failing the whole directory listing

vfs/sftpfs.go

@@ -427,6 +427,9 @@ func (*SFTPFs) IsNotExist(err error) bool {
 // IsPermission returns a boolean indicating whether the error is known to
 // report that permission is denied.
 func (*SFTPFs) IsPermission(err error) bool {
+	if _, ok := err.(*pathResolutionError); ok {
+		return true
+	}
 	return os.IsPermission(err)
 }
 
@@ -584,11 +587,11 @@ func (fs *SFTPFs) isSubDir(name string) error {
 	}
 	if len(name) < len(fs.config.Prefix) {
 		err := fmt.Errorf("path %#v is not inside: %#v", name, fs.config.Prefix)
-		return err
+		return &pathResolutionError{err: err.Error()}
 	}
 	if !strings.HasPrefix(name, fs.config.Prefix+"/") {
 		err := fmt.Errorf("path %#v is not inside: %#v", name, fs.config.Prefix)
-		return err
+		return &pathResolutionError{err: err.Error()}
 	}
 	return nil
 }