minor defect: missing invalidation or PResult

To have consistent results, PResult must be invalidated when loading/saving.
In fact, there is no invalidation when saving at all and also there is no invalidation, when loading from file and the stream cannot be opened. As a consequence, even if saving succeeds, there may be an error code from previous operation. Also if loading fails because the stream cannot be opened, then all information in PResult are irrelevant but error_code. Shall be 0 thus.

By the way, maybe the name PResult is historical: "ParseResult"? Maybe better: read/write result RWResult.