Return client_api error codes, even with json output enabled

Previously on error, the client API would return 0 (success) in the
cleanup and fail error handling path when outputting json to avoid
double-closing the JSON output. With iperf_json_finish now idempotent,
the client api can faithfully return an error when it fails, and the
calling context will not double-close the output if it has already
been closed.

This addresses #1405 .

src/iperf_client_api.c

@@ -708,10 +708,6 @@ iperf_run_client(struct iperf_test * test)
     if (test->json_output) {
         cJSON_AddStringToObject(test->json_top, "error", iperf_strerror(i_errno));
         iperf_json_finish(test);
-        iflush(test);
-        // Return 0 and not -1 since all terminating function were done here.
-        // Also prevents error message logging outside the already closed JSON output.
-        return 0;
     }
     iflush(test);
     return -1;