This is part of TLS 1.3 draft 16 but isn't much of a wire format change,
so go ahead and add it now. When rolling into Chromium, we'll want to
add an entry to the error mapping.
Change-Id: I8fd7f461dca83b725a31ae19ef96c890d603ce53
Reviewed-on: https://boringssl-review.googlesource.com/11563
Reviewed-by: David Benjamin <davidben@google.com>