Bugfix: Send OFPT_ERROR, not OFPT_FEATURES_REQUEST

Send OFPT_ERROR (with type field of OFPET_HELLO_FAILED,
and code field of OFPHFC_INCOMPATIBLE) for error when
version negotiation fails, not OFPT_FEATURES_REQUEST.
Merge pull request #3 from bertvl-netro/of_handshake_fix

Bugfix: Send OFPT_ERROR, not OFPT_FEATURES_REQUEST