Implement Unwrap() on errors returned from rulefmt

I'd like to unwrap errors returned from rulefmt but both Error and WrappedError types are missing Unwrap() method.

Signed-off-by: Łukasz Mierzwa <l.mierzwa@gmail.com>

model/rulefmt/rulefmt.go

@@ -48,6 +48,11 @@ func (err *Error) Error() string {
 	return errors.Wrapf(err.Err.err, "group %q, rule %d, %q", err.Group, err.Rule, err.RuleName).Error()
 }
 
+// Unwrap unpacks wrapped error for use in errors.Is & errors.As.
+func (err *Error) Unwrap() error {
+	return &err.Err
+}
+
 // WrappedError wraps error with the yaml node which can be used to represent
 // the line and column numbers of the error.
 type WrappedError struct {
@@ -66,6 +71,11 @@ func (we *WrappedError) Error() string {
 	return we.err.Error()
 }
 
+// Unwrap unpacks wrapped error for use in errors.Is & errors.As.
+func (we *WrappedError) Unwrap() error {
+	return we.err
+}
+
 // RuleGroups is a set of rule groups that are typically exposed in a file.
 type RuleGroups struct {
 	Groups []RuleGroup `yaml:"groups"`

model/rulefmt/rulefmt_test.go

@@ -15,6 +15,7 @@ package rulefmt
 
 import (
 	"errors"
+	"io"
 	"path/filepath"
 	"testing"
 
@@ -299,3 +300,27 @@ func TestWrappedError(t *testing.T) {
 		})
 	}
 }
+
+func TestErrorUnwrap(t *testing.T) {
+	err1 := errors.New("test error")
+
+	tests := []struct {
+		wrappedError   *Error
+		unwrappedError error
+	}{
+		{
+			wrappedError:   &Error{Err: WrappedError{err: err1}},
+			unwrappedError: err1,
+		},
+		{
+			wrappedError:   &Error{Err: WrappedError{err: io.ErrClosedPipe}},
+			unwrappedError: io.ErrClosedPipe,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.wrappedError.Error(), func(t *testing.T) {
+			require.ErrorIs(t, tt.wrappedError, tt.unwrappedError)
+		})
+	}
+}