code: func SetData(key DataKey, value interface{}) ErrorOption {
  	return func(m map[DataKey]interface{}) {
  		m[key] = value
  	}
  }
docs: // SetData will take the given value and store it with the error or error class
  // and its descendents associated with the given DataKey. Be sure to check out
  // the example. value can be nil to disable values for subhierarchies.