Revert "extenum returns ~uint64_t(0) on failure"
Revert "extenum returns ~uint64_t(0) on failure"

This reverts commit 9d0a0b87c61907b23f8b7f68ee9644903acf53cc.

fplll/fplll,dstehle/fplll,dstehle/fplll,cr-marcstevens/fplll,fplll/fplll,fplll/fplll,fplll/fplll,dstehle/fplll,cr-marcstevens/fplll,cr-marcstevens/fplll,fplll/fplll,cr-marcstevens/fplll