message: doc: elaborate error message when --genes are given without gene map
diff: @@ -33,7 +33,7 @@ pub fn read_gene_map(input_gene_map: &Option<PathBuf>, genes: &Option<Vec<String
  (Some(input_gene_map), None) => read_gff3_file(&input_gene_map),
  // Gene list is provided, but no gene map. This is illegal.
  - (None, Some(_)) => make_error!("List of genes is only valid when gene map is provided"),
  + (None, Some(_)) => make_error!("List of genes via '--genes' can only be specified when a gene map (genome annotation) is provided"),
  // Nothing is provided. Create an empty gene map.
  // This disables codon-aware alignment, translation, AA mutations, frame shifts, and everything else that relies
  