Fix behavior of sleuth_results when gene_mode is TRUE (and error reporting)

[warrenmcg]

Hi @pimentel,

This may not be relevant with gene_mode hidden now, but I noticed that I had previously based adding gene metadata when gene_mode = TRUE only when show_all = TRUE, which isn't technically correct. I fixed this behavior.

I also took this opportunity to improve the error reporting for the case when pval_aggregation is requested, but a target_mapping is missing, to make clear to the user what they need to do.

[pimentel]

fantastic, thanks!