iproto: rename IPROTO_ERROR and IPROTO_ERROR_STACK

IPROTO_ERROR in fact is not an error. It is an error message.
Secondly, this key is deprecated in favor of IPROTO_ERROR_STACK,
which contains all attributes of the whole error stack. It uses
MP_ERROR MessagePack extenstion for that.

So IPROTO_ERROR is renamed to IPROTO_ERROR_24 (similar to how old
call was renamed to IPROTO_CALL_16). IPROTO_ERROR_STACK becomes
new IPROTO_ERROR.

Follow up #4398