title: Ant fail if filter token not present or token not replaced
body: <p>In Ant, does anyone know of a concise way to go about having a <code>build.xml</code> fail if, when copying a file, token replacement for a particular token does not occur. I.e., the token was not found in the file, and thus was not replaced.</p>
  
  <p>The idea behind this is that in certain circumstances it may be crucial for a tokenised file to have certain tokens replaced, and so it's worth validating that those tokens exist in the file to begin with - by default Ant would just do nothing if the token isn't there.</p>
  
  <p>I've looked at the docs for <code>filter</code> and <code>filterset</code> but they don't seem to have an attribute for failing if the token searched for is not present.</p>
  
  <p>Perhaps this could be done with a regex task to check if the token is present. Are there any neater ways people can think of?</p>