OAuthCallbackController_2.cls

public OAuthCallbackController(){
  String error = ApexPages.currentPage().getParameters().get('error');
  String errorDescription = ApexPages.currentPage().getParameters().get('error_description');

  if(String.isEmpty(error)){
    requestToken();
  }else{
    ApexPages.addMessage(
      new ApexPages.Message(
        ApexPages.Severity.ERROR, 'error:' + error + ' description:' + errorDescription
      )
    );
  }
}