title: Could not find migration class for migration step symfony doctrine
body: <p>I am trying to migrate back to a previous step, I have accidentally deleted my current migration step '56'. When I run</p>
  
  <pre><code>php symfony doctrine:migrate 51
  </code></pre>
  
  <p>As expected, it throws an error "<code>Could not find migration class for migration step: 56</code>" -- this is due to me deleting the migration 56 class</p>
  
  <p>What is the solution to roll back to the previous step "51", without having the current class already there</p>
  
  <p>I am using an older version of symfony v 1.2</p>