restore some old decode-error field handling behaviour

When some rework around decoding/encoding certain types was optimised in PROTON-1708 away from using the more generic type handling, to using very frame-specific handling for certain hot cases (e.g Flow, Transfer, Disposition), some of the outer behaviours around mandatory fields changed. Some additional checks/handling the older type code did should be added to the newer bits to restore those behaviours, which can make things clearer when certain decode issues occur.