ofp-errors: Add (draft) OpenFlow 1.5 group mod errors.

Add OFPERR_OFPGMFC_UNKNOWN_BUCKET and OFPERR_OFPGMFC_BUCKET_EXISTS
which are part of (draft) OpenFlow 1.5 groups.

ONF-JIRA: EXT-350
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>