Print readable error messages in doc code blocks When parsing fails inside a `{@ocaml[...]}` block, the catch-all branch formatted the exception with `Exn.pp`, which for `Lexer.Error` produced the raw constructor pattern `Ocamlformat_parser_extended.Lexer.Error(_, _)`. `Lexer.Error` (and most other compiler-libs exceptions) register a printer via `Location.register_error_of_exn`, but that table is separate from the `Printexc` table `Exn.pp` consults, so the printer was never used. Route the catch-all through `Location.error_of_exn` instead: if a printer is registered, format its `main` message; otherwise fall back to `Exn.pp`. This handles `Lexer.Error` and any other registered exception (e.g. parser internals) without per-variant enumeration. The .err diff against the previous commit shows the before/after for the six variants the regression test exercises.