Distinguish FwdState::fail() callers

Distinguish FwdState::fail() callers

The fail() method performs two actions, saving an error and making some
cleanup operations. However, in reality the most of the callers do only
the first step because the second (cleanup) depends on a rare
ERR_ZERO_SIZE_OBJECT condition. We could probably optimize all these
callers, calling saveError() instead of fail() - but this can cause some
difficulties in future if the cleanup condition changes. However, I
think it is reasonable to do this for internal callers only (i.e., in
FwdState itself).