tracerr.go
func (t *traceableError) Error() string {
	str := t.err.Error()
	for _, frame := range t.stack {
		str += fmt.Sprintf("\n  at %s", frame.string())
	}
	return str
}
func (t *traceableError) Error() string {
	str := t.err.Error()
	for _, frame := range t.stack {
		str += fmt.Sprintf("\n  at %s", frame.string())
	}
	return str
}
// Error returns the original error message plus the stack trace captured
// at the time the error was first wrapped.