title: how to validate bing maps api key?
body: <p>I am using the leaflet bing maps plugin.
  How can I validate the bing maps api key before it's used by leaflet?</p>
  
  <p>If I allow an invalid key to be used like this:</p>
  
  <pre><code>var bing_key = "funTimeWithBingMaps"
  baseMapUrl = new L.BingLayer(bing_key)
  </code></pre>
  
  <p>Then bing maps reports:</p>
  
  <p>"Leaflet Bing Plugin Error - Got metadata: Access was denied. You may have entered your credentials incorrectly, or you might not have access to the requested resource or operation." </p>
  
  <p>And then <code>map.removeLayer(baseMapUrl);</code> fails to remove the layer.</p>