lexer: refactor Error.Error()'s prefix generator

Signed-off-by: Alejandro Mery <amery@jpi.io>

lexer/error.go

@@ -28,14 +28,25 @@ type Error struct {
 	Err     error
 }
 
+func (err Error) prefix() string {
+	switch {
+	case err.Line > 0 || err.Column > 0:
+		if err.Filename != "" {
+			return fmt.Sprintf("%s:%v:%v", err.Filename, err.Line, err.Column)
+		}
+
+		return fmt.Sprintf("%v:%v", err.Line, err.Column)
+	default:
+		return err.Filename
+	}
+}
+
 func (err Error) Error() string {
 	var s []string
 
-	switch {
-	case err.Line > 0 || err.Column > 0:
-		s = append(s, fmt.Sprintf("%s:%v:%v", err.Filename, err.Line, err.Column))
-	case err.Filename != "":
-		s = append(s, err.Filename)
+	prefix := err.prefix()
+	if prefix != "" {
+		s = append(s, prefix)
 	}
 
 	if err.Err != nil {