[serialize] Streamline error propagation

src/hb-buffer.hh

diff --git a/src/hb-buffer.hh b/src/hb-buffer.hh
index 0ac88c0..8a3170c 100644
--- a/src/hb-buffer.hh
+++ b/src/hb-buffer.hh
@@ -137,6 +137,8 @@
 
   /* Methods */
 
+  bool in_error () const { return !successful; }
+
   void allocate_var (unsigned int start, unsigned int count)
   {
 #ifndef HB_NDEBUG