Add external versioner (ref #573)

internal/versioner/simple.go

@@ -47,13 +47,12 @@ func NewSimple(folderID, folderPath string, params map[string]string) Versioner
 // nil, the named file does not exist any more (has been archived).
 func (v Simple) Archive(filePath string) error {
 	fileInfo, err := os.Lstat(filePath)
-	if err != nil {
-		if os.IsNotExist(err) {
-			if debug {
-				l.Debugln("not archiving nonexistent file", filePath)
-			}
-			return nil
+	if os.IsNotExist(err) {
+		if debug {
+			l.Debugln("not archiving nonexistent file", filePath)
 		}
+		return nil
+	} else if err != nil {
 		return err
 	}