title: Apache Camel redelivery: how to use the attempt number
body: <p>I have a Camel Route with the onException clause: at each redelivery I want to increase the redeliveryDelay. How do I get the attemptNumber?
  The DefaultErrorHandler clearly stores it somewhere because it prints it in the log e.g. "On delivery attempt: 1 caught"</p>
  
  <pre><code>onException(MyException.class)
      .handled(true)
      .log(LoggingLevel.ERROR, "Exception. Retry in " + (10000 * attemptNumber) + " ms")
      .maximumRedeliveries(2)
      .redeliveryDelay(10000 * attemptNumber)
  </code></pre>
  
  <p>Thank you!</p>