Rework error handling in the service protocol and in Observatory.

runtime/observatory/lib/src/elements/instance_ref.html

 <link rel="import" href="../../../../packages/polymer/polymer.html">
 <link rel="import" href="curly_block.html">
 <link rel="import" href="observatory_element.html">
 <link rel="import" href="service_ref.html">
 
 <polymer-element name="instance-ref" extends="service-ref">
   <template>
     <link rel="stylesheet" href="css/shared.css">
     <style>
       .errorBox {
         background-color: #f5f5f5;
         border: 1px solid #ccc;
         padding: 10px;
         font-family: consolas, courier, monospace;
         font-size: 1em;
         line-height: 1.2em;
         white-space: pre;
       }
     </style>
     <span>
       <template if="{{ ref.isSentinel }}">
-        <div title="{{ hoverText }}">{{ ref.valueAsString }}</div>
+        <span title="{{ hoverText }}">{{ ref.valueAsString }}</span>
       </template>
 
       <template if="{{ ref.isBool || ref.isInt ||
                        ref.isDouble || ref.isNull }}">
         <a on-click="{{ goto }}" _href="{{ url }}">{{ ref.valueAsString }}</a>
       </template>
 
       <template if="{{ ref.isString }}">
         <a on-click="{{ goto }}" _href="{{ url }}">{{ asStringLiteral(ref.valueAsString, ref.valueAsStringIsTruncated) }}</a>
       </template>
@@ skipping 74 matching lines @@
               </div>
             </div>
           </div>
         </curly-block>
       </template>
     </span>
   </template>
 </polymer-element>
 
 <script type="application/dart" src="instance_ref.dart"></script>