title: Visual studio code extension: How can I add error markers to files in the explorer?
body: <p>I have written a validator for my vscode extension, which uses a <code>DiagnosticCollection</code> to set errors for files. This works and the errors are shown when a file is opened.</p>
  <p>I would now like to mark files in the explorer, so that it is easy to find files with errors. Here is an example]of how it looks in Eclipse:</p>
  <p><img src="https://help.eclipse.org/latest/topic/org.eclipse.jdt.doc.user/gettingStarted/images/workbench_testcase_errors.png" alt="" /></p>
  <p>Is it possible to do this in a vscode extension? Is there already an extension doing this?</p>