Special error to indicate that an optional field is missing.

The return values of a check must be as follows: Method can be correctly executed, and the condition checked is true: - return &tBool{value: true}, nil Method can be correctly executed, but the condition checked is false: - return &tBool{value: false}, error("message indicating why the check failed") Method cannot be correctly executed (the arguments are invalid): - return nil, error("error message").