title: using guard to check for nil without implicitly unwrapping
body: <p>I know there are some similar questions around, but I couldn't find one specific to my issue. 
  I have a request where I want to check for the presence of the <code>error</code> key. it is not present everything is fine, if not I should handle the error. Currently, I have it implemented as follows:</p>
  
  <pre><code>if let error = json["error"] {
      // handle error
  }
  else {
      // handle success
  }
  </code></pre>
  
  <p>I would like to use a guard statement here to have the success case unindented. The only way I came up with is</p>
  
  <pre><code>guard json["error"] == nil else {
      let error = json["error"]!
      // handle error
  } 
  
  // handle success
  </code></pre>
  
  <p>but that seems wrong to me with the !. Are there any other approaches to this?</p>