Add Media Element error reporting.

This allows us to report an error in cases such as the user agent not supporting encrypted media and tries to decode the encrypted file.
diff --git a/adaptive/dash-player.html b/adaptive/dash-player.html
index ef96786..07abf10 100644
--- a/adaptive/dash-player.html
+++ b/adaptive/dash-player.html
@@ -173,6 +173,10 @@
   clearChunkDrawBackground = true;
 }
 
+function onMediaError(e) {
+  alert('Encountered MediaError ' + e.target.error.code);
+}
+
 /**
  * Renders chunks downloaded by stream and duration to a canvas.
  * @param {Canvas} c The canvas for rendering.
@@ -273,7 +277,7 @@
 </head>
   <body>
     <input id="u" type="text" size="80" value="webm_dash_mevq-key-enc-vid6-aud1.mpd.xml" /><input type="button" value="Start" onClick="onStartClick();"><br/>
-    <video id="v" autoplay>
+    <video id="v" autoplay onerror="onMediaError(event)">
     </video>
 
     <hr>