caddyfile: keep error chain info in Dispenser.Errf (#4233)

* caddyfile: Errf enable error chain unwrapping

* refactor: remove parseError

caddyconfig/caddyfile/dispenser.go

@@ -345,13 +345,13 @@ func (d *Dispenser) EOFErr() error {
 
 // Err generates a custom parse-time error with a message of msg.
 func (d *Dispenser) Err(msg string) error {
-	msg = fmt.Sprintf("%s:%d - Error during parsing: %s", d.File(), d.Line(), msg)
-	return errors.New(msg)
+	return d.Errf(msg)
 }
 
 // Errf is like Err, but for formatted error messages
 func (d *Dispenser) Errf(format string, args ...interface{}) error {
-	return d.Err(fmt.Sprintf(format, args...))
+	err := fmt.Errorf(format, args...)
+	return fmt.Errorf("%s:%d - Error during parsing: %w", d.File(), d.Line(), err)
 }
 
 // Delete deletes the current token and returns the updated slice

caddyconfig/caddyfile/dispenser_test.go

@@ -15,6 +15,7 @@
 package caddyfile
 
 import (
+	"errors"
 	"reflect"
 	"strings"
 	"testing"
@@ -303,4 +304,10 @@ func TestDispenser_ArgErr_Err(t *testing.T) {
 	if !strings.Contains(err.Error(), "foobar") {
 		t.Errorf("Expected error message with custom message in it ('foobar'); got '%v'", err)
 	}
+
+	var ErrBarIsFull = errors.New("bar is full")
+	bookingError := d.Errf("unable to reserve: %w", ErrBarIsFull)
+	if !errors.Is(bookingError, ErrBarIsFull) {
+		t.Errorf("Errf(): should be able to unwrap the error chain")
+	}
 }