title: Rake clean raise error when the file does not exist
body: <p>I have a Rakefile like this</p>
  
  <pre><code>task :clean do
    sh 'rm ./foo'
  end
  </code></pre>
  
  <p>I want to prevent it from reporting error when the file 'foo' does not exist. How to do that?</p>
  
  <p>I think what I want is: Is there a way to check the file first and then decide what to do next. </p>
  
  <p>For example:</p>
  
  <pre><code>file 'aaa' =&gt; 'bbb' do
    sh 'cp bbb aaa'
  end
  </code></pre>
  
  <p>This task depends on the existence of file 'bbb', so I want to know <code>can I tell Rake that my task depends on the</code> <strong>non-existence</strong> <code>of file 'foo'</code> ?</p>