jsonrpc: Forward errors on malformed requests to cli

We were masquerading errors when parsing the request by reporting only
a bogus malformed `id` field in the response, when the real issue was
that we were unable to parse the request in the first place (which
caused the null-id error to be returned).

Fixes #4238

cli/lightning-cli.c

@@ -823,8 +823,9 @@ int main(int argc, char *argv[])
 		     "Missing 'id' in response '%s'", resp);
 	if (!json_tok_streq(resp, id, idstr))
 		errx(ERROR_TALKING_TO_LIGHTNINGD,
-		     "Incorrect 'id' in response: %.*s",
+		     "Incorrect 'id' (%.*s) in response: %.*s",
-		     json_tok_full_len(id), json_tok_full(resp, id));
+		     json_tok_full_len(id), json_tok_full(resp, id),
+		     json_tok_full_len(toks), json_tok_full(resp, toks));
 
 	if (!error || json_tok_is_null(resp, error)) {
 		switch (format) {

lightningd/jsonrpc.c

@@ -956,8 +956,10 @@ static struct io_plan *read_json(struct io_conn *conn,
 	if (!json_parse_input(&jcon->input_parser, &jcon->input_toks,
 			      jcon->buffer, jcon->used,
 			      &complete)) {
-		json_command_malformed(jcon, "null",
+		json_command_malformed(
-				       "Invalid token in json input");
+		    jcon, "null",
+		    tal_fmt(tmpctx, "Invalid token in json input: '%s'",
+			    tal_strndup(tmpctx, jcon->buffer, jcon->used)));
 		return io_halfclose(conn);
 	}