code: func AddError(ch *Changeset, field string, message string) {
  	ch.errors = append(ch.errors, errors.New(message, field, errors.Changeset))
  }
docs: // AddError adds an error to changeset.
  //	ch := changeset.Cast(user, params, fields)
  //	changeset.AddError(ch, "field", "error")
  //	ch.Errors() // []errors.Error{{Field: "field", Message: "error"}}