code: func WithValue(e error, key, value interface{}) Error {
  	if e == nil {
  		return nil
  	}
  	return WrapSkipping(e, 1).WithValue(key, value)
  }
docs: // WithValue adds a context an error.  If the key was already set on e,
  // the new value will take precedence.
  // If e is nil, returns nil.