caml_tex: also provide error output on Lexing errors

I hit a Lexer error by mistakenly closing an environment with

    \end{caml_example*}{verbatim}

instead of

    \end{caml_example*}

and caml_tex would not quote the wrong input or indicate
at which line the error was, which makes debugging painful.

tools/caml_tex.ml

@@ -196,7 +196,9 @@ module Toplevel = struct
           let str =
             Ast_helper.[Str.modtype @@ Mtd.mk ~typ:(Mty.signature sign) name] in
           Ptop_def str
-    with Syntaxerr.Error _ -> raise (Phrase_parsing s)
+    with
+    | Lexer.Error _ | Syntaxerr.Error _ ->
+        raise (Phrase_parsing s)
 
   let take x = let r = !x in x := []; r