Fix Client-certificate security

[hazen]

@macintux has modified riak_api to now actually return errors from SSL should they occur in basho/riak_api#66. The test_security_cert_authentication test now fails:

SecurityError: [('SSL routines', 'SSL3_READ_BYTES', 'tlsv1 alert unknown ca')]

Generating a new set of certs returns

SecurityError: [('SSL routines', 'SSL3_READ_BYTES', 'sslv3 alert certificate unknown')]

These map to the following C enums:

• SSL_R_TLSV1_ALERT_UNKNOWN_CA
• SSL_R_SSLV3_ALERT_CERTIFICATE_UNKNOWN

[hazen]

Resolved in #373