usnic: convert some BTL_ERRORs to more descriptive show_help messages

1. After we receive N abnormally-short messages (meaning: corrupted),
print a show_help message about it.  N defaults to 25.  N can be set
to 0 disable the message via btl_usnic_max_short_packets.
1. If we receive a completion error for something other than a
receive, display a show_help message.

Reviewed by Dave Goodell.

CMR'ing to v1.8.3, but it will require a custom patch because of the
OMPI->OPAL BTL move.

cmr=v1.8.3