Raise usage error when parsing empty certificate

[goatgoose]

Problem:

Currently, s2n-tls will raise an internal error when parsing an empty certificate like the following:

-----BEGIN CERTIFICATE-----
-----END CERTIFICATE-----

This occurs when the s2n-tls PEM parser is invoked, in functions like s2n_cert_chain_and_key_load_pem().

Solution:

s2n-tls should instead return a usage error with a more helpful error message. We should also make sure that similar parsing issues result in usage errors.