title: Ant concat task complain on missing files
body: <pre><code>&lt;concat destfile="dist/external.js"&gt;
    &lt;fileset dir="."&gt;
      &lt;include name="a.js" /&gt;
      &lt;include name="b.js" /&gt;
    &lt;/fileset&gt;
  &lt;/concat&gt;
  </code></pre>
  
  <p>This won't fail if a or b is missing.  I've tried setting optional="false" or asis="true" and it always complains that those attributes don't exist.</p>