Improve some error messages if a digest is not available

Improve some error messages if a digest is not available

If a digest is not available we just get an "internal error" error
message - which isn't very helpful for diagnosing problems. Instead we
explicitly state that we couldn't find a suitable digest.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/12733)