usnic: improve error message

When sendto() fails, it would be helpful to see the errno value.

opal/mca/btl/usnic/btl_usnic_cagent.c

@@ -300,7 +300,10 @@ static void agent_sendto(int fd, char *buffer, ssize_t numbytes,
                 continue;
             }
 
-            ABORT("Unexpected sendto() error");
+            char *msg;
+            asprintf(&msg, "Unexpected sendto() error: errno=%d (%s)",
+                     errno, strerror(errno));
+            ABORT(msg);
             /* Will not return */
         }