Regular expression pattern to match error lines in the build log. 
For example:
<ul>
  <li><code>ERROR|error|Error</code> - matches lines containing any form of "error"</li>
  <li><code>FAILED|failed|Failed</code> - matches lines containing any form of "failed"</li>
  <li><code>Exception|exception</code> - matches lines containing exceptions</li>
</ul>
Leave empty to analyze all lines in the build log.