server: return real error from fio_send_data()

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/server.c b/server.c
index 1085180..a366f68 100644
--- a/server.c
+++ b/server.c
@@ -91,6 +91,9 @@
 	if (!len)
 		return 0;
 
+	if (errno)
+		return -errno;
+
 	return 1;
 }