Returns the error status of the buffer.

Return the length, whenever we return an array.  Makes it easier on the
language bindings.

This includes HB_DIRECTION_INVALID and HB_SCRIPT_INVALID.

The INVALID will cause a "guess whatever from the text" in hb_shape().
While it's not ideal, it works better than the previous defaults at
least (HB_DIRECTION_LTR and HB_SCRIPT_COMMON).

One should use hb_buffer_reset() really.