Check that SSL certificate is passed before redirecting to SSL authentication.

This is an enhancement to #1913 as a result of #1924.  It checks to make sure that your certificate information is available before redirecting the user to the ssl login.  As an important note, it converts the _ssl_get_cert_from_request to ssl_get_cert_from_request, and if merged before #1862 will require that PR to be rebased.

@sarina @ichuang @singingwolfboy