title: grunt uglify task failing
body: <p>When running grunt, I get the following error:</p>
  
  <blockquote>
    <p>Warning: Unable to write "client/dist/js/build.js" file (Error code: undefined). Use --force to continue.</p>
  </blockquote>
  
  <p>The config of uglify in my <code>Gruntfile.js</code> : </p>
  
  <pre><code>uglify: {
        build: {
          src: ['client/src/js/*.js'],
          dest:['client/dist/js/build.js']
        }
      }
  </code></pre>
  
  <p>I'm using <code>grunt-contrib-uglify</code>.</p>
  
  <p>Any ideas why this is happening?</p>