EVP_Digest*Update, EVP_DigestFinal, and HMAC_Update can never fail.

Enough code fails to check their return codes anyway. We ought to make
it official.

Change-Id: Ie646360fd7073ea943036f5e21bed13df7e1b77a
Reviewed-on: https://boringssl-review.googlesource.com/4954
Reviewed-by: Adam Langley <agl@google.com>