title: What is this "err.(*exec.ExitError)" thing in Go code?
body: <p>For instance, in this answer: </p>
  
  <p><a href="https://stackoverflow.com/a/10385867/20654">https://stackoverflow.com/a/10385867/20654</a></p>
  
  <pre><code>...
  if exiterr, ok := err.(*exec.ExitError); ok {
  ...
  </code></pre>
  
  <p>What is that <code>err.(*exec.ExitError)</code>  called? How does it work?</p>