use Arg.bad instead of Failure exception when positional atomic number arguments

elements.ml

@@ -32,7 +32,10 @@ module Opts = struct
 	let add_def s = match !ltype with
 	| NAME -> add_name s
 	| SYMB -> add_symb s
-	| NUMB -> add_numb (int_of_string s)
+	| NUMB -> add_numb (match int_of_string_opt s with
+		| None -> raise (Bad (Printf.sprintf "expected integer, got '%s'" s))
+		| Some x -> x
+	)
 
 	let msg = "Periodic table search\n\
 		elements -gendb file [-dbfile outfile]\n\