BUG/MINOR: ssl/lua: use correctly cert_ext in CertCache.set()

BUG/MINOR: ssl/lua: use correctly cert_ext in CertCache.set()

Fix a typo that lead to using the wrong pointer when loading a
certificate, which lead to always using the pem loader for every
parameeter.

Use the cert_ext->load() ptr instead of cert_exts->load() which was the
first element of the cert_exts[] array.

Enhance the error message with the field name.

Should fix issue #1716