Improve error message when there is limited connectivity

[mmc46]

When there is limited connectivity, cf_cosine_plot show an error message saying there are no valid IDs. it would be useful if the error message was more specific, as the IDs are valid.

cf_ids(malecns = "type:MNad43", expand = T)
malecns [2 ids]: 812354 807837

cf_cosine_plot(cf_ids(malecns = "type:MNad43"))
Error in neuprintr::neuprint_ids(x = x, conn = conn, mustWork = mustWork,  : 
  No valid ids provided!

cf_cosine_plot(cf_ids(malecns = c(812354,807837)))
Error in neuprintr::neuprint_ids(x = x, conn = conn, mustWork = mustWork,  : 
  No valid ids provided!