Return NGHTTP2_ERR_BUFFER_ERROR from nghttp2_hd_{deflate,inflate}_hd

It is generally useful to know what is the cause of the error.  Since
we expose HPACK API, it is friendly to tell application the
insufficient buffer size is a culprit.

lib/nghttp2_int.h

@@ -45,7 +45,6 @@ typedef int (*nghttp2_compar)(const void *lhs, const void *rhs);
    inclusive. */
 typedef enum {
   NGHTTP2_ERR_CREDENTIAL_PENDING = -101,
-  NGHTTP2_ERR_BUFFER_ERROR = -102,
   NGHTTP2_ERR_IGN_HEADER_BLOCK = -103,
   NGHTTP2_ERR_IGN_PAYLOAD = -104
 } nghttp2_internal_error;