Rename in_error to !successful

Towards possibly using Null pool for some nil objects.

src/hb-ot-shape-complex-thai.cc

diff --git a/src/hb-ot-shape-complex-thai.cc b/src/hb-ot-shape-complex-thai.cc
index 5902b36..02d78ac 100644
--- a/src/hb-ot-shape-complex-thai.cc
+++ b/src/hb-ot-shape-complex-thai.cc
@@ -315,7 +315,7 @@
 
   buffer->clear_output ();
   unsigned int count = buffer->len;
-  for (buffer->idx = 0; buffer->idx < count && !buffer->in_error;)
+  for (buffer->idx = 0; buffer->idx < count && buffer->successful;)
   {
     hb_codepoint_t u = buffer->cur().codepoint;
     if (likely (!IS_SARA_AM (u))) {
@@ -327,7 +327,7 @@
     hb_codepoint_t decomposed[2] = {hb_codepoint_t (NIKHAHIT_FROM_SARA_AM (u)),
 				    hb_codepoint_t (SARA_AA_FROM_SARA_AM (u))};
     buffer->replace_glyphs (1, 2, decomposed);
-    if (unlikely (buffer->in_error))
+    if (unlikely (!buffer->successful))
       return;
 
     /* Make Nikhahit be recognized as a ccc=0 mark when zeroing widths. */