Fix s_server app to not report an error when using a non DH certificate.

Fixes #15071

It always tries loading the cert as DH which previously did not produce
an error. The errors are not suppressed for these operations.
The output now matches previous versions of OpenSSL.

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15670)