Fix error message for port_mod to non-existent port

Return OFPET_PORT_MOD_FAILED + OFPPMFC_BAD_PORT instead of
OFPET_BAD_REQUEST + OFPBRC_BAD_PORT when the port number is valid but
the port doesn't exist.

Keep sending OFPET_BAD_REQUEST + OFPBRC_BAD_PORT for requests with
invalid or reserved port numbers.

This closes FlowForwarding/LINC-Switch#238.
Merge pull request #255 from legoscia/port-mod-error

Fix error message for port_mod to non-existent port