Instead, just act as though the certificate file(s) are not present.
There is only one case where this need be a hard failure condition: when
sslmode is verify-ca or verify-full, not having a root cert file is an
error.  Change the logic so that we complain only in that case, and
otherwise fall through cleanly.  This is how it used to behave pre-9.0,
but my patch 4ed4b6c5 of 2010-05-26 broke
the case.  Per report from Christian Kastner.