row indices of NA observations in error message

[idot]

adds indices of genes failing filtering to error message

[drisso]

Hi @idot ,

thanks for the PR and sorry for not responding earlier, I hope this is still relevant and you are still willing to contribute.

Your PR would not work if pos_controls is a character vector. Can you please edit it so that it would? Also, it would be good to avoid 1:nrow(expr) and use seq_len(nrow(expr)) instead.