toolbox/Utils.translate_nucleotide_sequence_whatever_it_takes: more failsafe

lib/toolbox/utils.ml

@@ -26,7 +26,7 @@ let int_for_all a b ~f =
   in
   loop a
 
-let translate_nucleotide_sequence_whatever_it_takes ?(unknown_char = 'X') seq =
+let translate_nucleotide_sequence_whatever_it_takes ?(unknown_char = '-') seq =
   let open Phylogenetics in
   let has_some_gaps s = String.exists s ~f:(Char.( = ) '-') in
   let n = String.length seq in
@@ -40,7 +40,5 @@ let translate_nucleotide_sequence_whatever_it_takes ?(unknown_char = 'X') seq =
         )
       | None ->
         if has_some_gaps subseq then '-'
-        else if String.exists subseq ~f:(Char.( = ) 'N') then unknown_char
-        else
-          failwithf "Sequence %s cannot be converted to aminoacid sequence because it contains %s" seq subseq ()
+        else unknown_char
     )