ofproto: Split add_flow().

Split add_flow() to add_flow_begin() which does all the error
checking, and add_flow_finish() which can not fail.

Since we still want to send an error response for an unknown
'buffer_id', send_buffered_packet() now send the error response
itself.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>