Improve error reporting when serializing non-Unicode strings to JSON

Improve error reporting when serializing non-Unicode strings to JSON

* src/coding.c (utf8_string_p): New helper function.
(syms_of_coding) <utf-8-unix>: Move from json.c.

* src/json.c (json_check_utf8): New helper function.
(lisp_to_json_toplevel_1, lisp_to_json): Use it.  To save a bit of
time, check for invalid UTF-8 strings only after encountering an
error, since Jansson already rejects them.

* test/src/json-tests.el (json-serialize/invalid-unicode): Adapt
expected error symbol.