Better Error Messages for OpenSSL exceptions related to ca_cert_path

[mwrock]

I have seen on a couple occasions where users are confused when trying to use a *.pfx file as the ca_cert_path (see chef/knife-windows#315). This gem simply raises the OpenSSL CertStore error which is not very informative. The Store error should be rescued and a more actionable error should be raised instructing the user on the valid cert formats accepted.