sax_parse() recovers from an error if parse_error() returns true. For a
handler whose parse_error() is [[noreturn]], MSVC reports that code as
unreachable (C4702), which fails the /WX module build. throwing_sax now
throws under a flag, as json_sax_dom_parser does.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>