Fix incorrect phase seeking in vt data reader

[ppebay]

In the definition of _populate_rank(), the following block:

# Seek phase with given ID
        phase_id_found = False
        for phase in self.__vt_data.get(rank_id).get("phases"):
            if (curr_phase_id := phase["id"]) != phase_id:
                # Ignore phases that are not of interest
                self.__logger.debug(
                    f"Ignored phase {curr_phase_id} for rank {rank_id}")
                continue
            # Desired phase was found
            phase_id_found = True
            break

will result in an incorrect phase if the ID was not found.

This must be protected by a final test on phase_id_found and result in an error (and exit) if not found.