usnic: do not disqualify if a peer does not put usnic modex info

If ompi_modex_recv() fails with OPAL_ERR_DATA_VALUE_NOT_FOUND, it
simply means that the peer process did not put any usnic BTL modex
info -- it is not an error.  So have the usnic BTL simply ignore that
peer (vs. disqualifying itself / treating this like a real error).

Refs #4442.