remove a questional error handling

as our underlying transport (tcp/ssl) is a stream which doesn't preserve
message boundaries, dropping an unparsable segment likely make the
situation worse.
fix message header parsing

if not enough bytes of data are available, return {error, binary_too_small}
so that the caller can retry when more data come.  it's normal as
our underlying transport (tcp etc) does not preserve message boundaries.
fix some of ofp_v4 table_feature_prop_type constants
uncut_bits: comment
fix crashes with non openflow_basic match fields
change oxm on-wire length for ipv6_flabel and mpls_label

this is doubtful per spec but matches with ONF openflow.h and
other implementations.  (ovs, ryu, ...)