You can show errors on a dashboard once you have ingested the messages and parsed them/extracted the required information.
Since you only have plaintext logs, you have to send them to Graylog (e. g. with Filebeat, nxlog, or Logstash) to an input which has the necessary extractors (see http://docs.graylog.org/en/2.2/pages/extractors.html and http://docs.graylog.org/en/2.2/pages/pipelines.html).