scp: return better error messages

Fixes #171

vfs/osfs.go

@@ -183,6 +183,16 @@ func (OsFs) IsPermission(err error) bool {
 	return os.IsPermission(err)
 }
 
+// GetPermissionError returns a permission error for this FS
+func (OsFs) GetPermissionError() error {
+	return os.ErrPermission
+}
+
+// GetNotExistError returns a not exist error for this FS
+func (OsFs) GetNotExistError() error {
+	return os.ErrNotExist
+}
+
 // CheckRootPath creates the root directory if it does not exists
 func (fs OsFs) CheckRootPath(username string, uid int, gid int) bool {
 	var err error