Double exception logging causes double stack trace #179

https://github.com/requests/requests-kerberos/blob/master/requests_kerberos/kerberos_.py#L232
This bit of error handling is logging the stack trace twice.

Perhaps one should be log.error() - or maybe both should be log.error() and leave it up to the caller to log the stack trace.