ofp-print: Print the type of truncated messages, when available.

The function ofputil_decode_msg_type_partial() can figure out the type of
a truncated OpenFlow message, if the message is long enough that its type
can be determined, so we should print this information.   This is
especially valuable for printing OFPT_ERROR messages, in which the inner
OpenFlow message is often truncated to 64 bytes.

Signed-off-by: Ben Pfaff <blp@nicira.com>