title: Get NUnit assertion failure message
body: <p>I'm pretty new in testing. After some simple exercises with Visual Studio I have turned to Nunit test framework. What I'm looking for is a property or a method to use inside a <code>[TearDown]</code> test method that provides me a string that contains, in failure case, the error message.
  For example if I code this assertion:</p>
  
  <pre><code>Assert.That(true, Is.False);
  </code></pre>
  
  <p>then the framework gets this error message:</p>
  
  <pre><code>Message: Expected: False But was: True
  </code></pre>
  
  <p>What I need is the string above.
  Thanks. </p>