title: rspec: raise_error usage to match error message
body: <p>I raised an error using <code>raise(ConfigurationError.new(msg))</code></p>
  
  <p>I tried to test this with rspec: </p>
  
  <pre><code>expect {
    Base.configuration.username
  }.to raise_error(ConfigurationError, message) 
  </code></pre>
  
  <p>But this doesn't work. How can I test this? The goal is to match <code>message</code>.</p>