Encoding of non-UTF-8 `String _ values, fail earlier?

[smondet]

Encoded arbitrary strings can fail to be parsed back:

# let teeshow s = Printf.printf "-> %S\n%!" s; s;;
val teeshow : string -> string = <fun>
# let x = Ezjsonm.(value_from_string (value_to_string (`String "\xbe") |> teeshow));;
-> "\"\190\""
Exception:
Ezjsonm.Parse_error (`Null,
 "JSON.of_buffer illegal bytes in character stream (BE)").

In value_to_src `String _ Ezjsonm.values are passed directly to Jsonm.encode e (`Lexeme l).

The Jsonm documentation says:

Only the UTF-8 encoding scheme is supported. The strings of encoded `Name and `String lexemes are assumed to be immutable and must be UTF-8 encoded, this is not checked by the module.

Should Ezjsonm fail earlier, at encoding time?