util/vizerror: add func WithInternal, Error.InternalError accessor

Updates tailscale/corp#23781

Change-Id: I072d0169703aefb0f37b1fa715881f1ccb561f03
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/vizerror/vizerror.go

@@ -13,6 +13,7 @@
 // Error is an error that is safe to display to end users.
 type Error struct {
 	err         error
+	internalErr error
 }
 
 // Error implements the error interface.
@@ -20,14 +21,21 @@ func (e Error) Error() string {
 	return e.err.Error()
 }
 
+// InternalError returns the internal error that should not be shown to end
+// users. It will be nil if the user-visible error was not constructed using
+// WithInternal.
+func (e Error) InternalError() error {
+	return e.internalErr
+}
+
 // New returns an error that formats as the given text. It always returns a vizerror.Error.
 func New(text string) error {
-	return Error{errors.New(text)}
+	return Error{err: errors.New(text)}
 }
 
 // Errorf returns an Error with the specified format and values. It always returns a vizerror.Error.
 func Errorf(format string, a ...any) error {
-	return Error{fmt.Errorf(format, a...)}
+	return Error{err: fmt.Errorf(format, a...)}
 }
 
 // Unwrap returns the underlying error.
@@ -36,11 +44,15 @@ func (e Error) Unwrap() error {
 }
 
 // Wrap wraps err with a vizerror.Error.
+//
+// Deprecated: this is almost always the wrong thing to do. Are you really sure
+// you know exactly what err.Error() will stringify to and be safe to show to
+// users?
 func Wrap(err error) error {
 	if err == nil {
 		return nil
 	}
-	return Error{err}
+	return Error{err: err}
 }
 
 // As returns the first vizerror.Error in err's chain.
@@ -48,3 +60,10 @@ func As(err error) (e Error, ok bool) {
 	ok = errors.As(err, &e)
 	return
 }
+
+// WithInternal returns a new ErrorWithInternal combining a user-visible error
+// string and an internal error to pass around for internal logging but not
+// to be shown to end users.
+func WithInternal(visibleError string, internalErr error) error {
+	return Error{err: errors.New(visibleError), internalErr: internalErr}
+}