Add more tests (and fix some things) for replacecommand

utils/replacecommand.go

@@ -36,7 +36,7 @@ func ExecuteModifications(modifications []ReplaceCommand, fileData string) (stri
 func (m *ReplaceCommand) Execute(fileDataStr string) (string, error) {
 	err := m.Validate(len(fileDataStr))
 	if err != nil {
-		return "", fmt.Errorf("failed to validate modification: %v", err)
+		return fileDataStr, fmt.Errorf("failed to validate modification: %v", err)
 	}
 
 	logger.Trace("Replace pos %d-%d with %q", m.From, m.To, m.With)
@@ -50,5 +50,8 @@ func (m *ReplaceCommand) Validate(maxsize int) error {
 	if m.From > maxsize || m.To > maxsize {
 		return fmt.Errorf("command from or to is greater than replacement length: %v", m)
 	}
+	if m.From < 0 || m.To < 0 {
+		return fmt.Errorf("command from or to is less than 0: %v", m)
+	}
 	return nil
 }