Improve error handling with Kerberos errors

[wooldridge]

The following is insufficient in the case of a Kerberos error under the hood:

kerberos.authGSSClientInit(uri, 0, function(err, ctx) {
    if (err) {
      operation.errorListener('kerberos initialization failed at '+uri);
    }
    operation.logger.debug('kerberos initialized at '+uri);
    kerberos.authGSSClientStep(ctx, "", function (err) {
      if (err) {
        operation.errorListener('kerberos credentials failed');
      }

Need to at least display the err object. This was apparent in #343 (comment) when I had to console.log() the authGSSClientStep err object to figure out what was going on.