`sleuth_gene_table` gives non-sensical results when `gene_mode = TRUE`

[warrenmcg]

Hi @pimentel,

This issue just came up on the forum, where a user was trying to use the sleuth_gene_table function with gene_mode = TRUE. It is intended for when gene_mode = FALSE, so it should have included a sanity check for the user. We also need to update its documentation, since the column names do not match what's listed.