ofproto: Report correct error for emergency flow with timeout.

ofproto: Report correct error for emergency flow with timeout.

The OpenFlow 1.0 specification says:

    Emergency flow mod messages must have timeout values set to zero.
    Otherwise, the switch must refuse the addition and respond with an
    ofp_error_msg with OFPET_FLOW_MOD_FAILED type and
    OFPFMFC_BAD_EMERG_TIMEOUT code.

but Open vSwitch reported OFPFMFC_TABLE_FULL in this case.  This commit
fixes the problem.

Fixes detailed_contr_sw_messages.EmerFlowTimeout failure in OFTest.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Kyle Mestery <kmestery@cisco.com>