Rework error handling in the service protocol and in Observatory.

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

 <link rel="import" href="../../../../packages/polymer/polymer.html">
 <link rel="import" href="instance_ref.html">
 <link rel="import" href="error_ref.html">
 <link rel="import" href="observatory_element.html">
 
 <polymer-element name="eval-box" extends="observatory-element">
   <template>
     <style>
       .textbox {
         flex-grow: 1;
@@ skipping 56 matching lines @@
 
     <br>
     <template if="{{ results.isNotEmpty }}">
       <table>
         <tr template repeat="{{ result in results }}">
           <td class="historyExpr">
             <a class="expr" on-click="{{ selectExpr }}"
                expr="{{ result['expr'] }}">{{ result['expr'] }}</a>
           </td>
           <td class="historyValue">
-            <template if="{{ result['value'] == null }}">
-              <div style="color:#aaa;cursor:wait;">&lt;pending&gt;</div>
+            <template if="{{ result['error'] != null}}">
+              <div style="color:red">{{ result['error'] }}</div>
             </template>
-            <template if="{{ result['value'] != null }}">
-              <any-service-ref ref="{{ result['value'] }}"></any-service-ref>
+            <template if="{{ result['error'] == null}}">
+              <template if="{{ result['value'] != null }}">
+                <any-service-ref ref="{{ result['value'] }}"></any-service-ref>
+              </template>
+              <template if="{{ result['value'] == null }}">
+                <div style="color:#aaa;cursor:wait;">&lt;pending&gt;</div>
+              </template>
             </template>
           </td>
         </tr>
       </table>
     </template>
   </template>
 </polymer-element>
 
 <script type="application/dart" src="eval_box.dart"></script>