*nghttp2_on_data_chunk_recv_callback return code not handled as documented

[ralphdolmans]

Hi. First of all, thanks for this work!

While working with this code is noticed some unexpected behavior, which is caused by a difference between documentation and code. Not sure which of the two are wrong.

According to https://nghttp2.org/documentation/types.html#c.nghttp2_on_data_chunk_recv_callback

The implementation of this function must return 0 if it succeeds. If nonzero is returned, it is treated as fatal error, and nghttp2_session_recv() and nghttp2_session_mem_recv() functions immediately return NGHTTP2_ERR_CALLBACK_FAILURE.

In reality, this is not the case for all nonzero values, only return values below NGHTTP2_ERR_FATAL (-900) are treated as fatal:

nghttp2/lib/nghttp2_session.c

Line 6575 in ef41583

if (nghttp2_is_fatal(rv)) {