Fix T49412: We do not support FBX 7500 and above, at least properly report it to users.

Those ... people? at AD changed the whole format on binary level it'd seem, even low-level,
binary parsing is broken with those files, nothing else to do but go back to
binary hacking/inspection of new files if we want to support them... will let that
to someone else, FBX has successfully exhausted my patience since years already,
even all the backup emergency reserves I had.