Check max SETTINGS_HEADER_TABLE_SIZE in nghttp2_iv_check()

Hide NGHTTP2_MAX_HEADER_TABLE_SIZE from public API.  Now it is defined
as ((1u << 31) - 1) in nghttp2_frame.h, which is sufficiently big
enough.
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.
Add missing library error code to nghttp2_error()
Remove outdated comment