Rework error handling in the service protocol and in Observatory.

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

Index: runtime/observatory/lib/src/elements/eval_box.dart
diff --git a/runtime/observatory/lib/src/elements/eval_box.dart b/runtime/observatory/lib/src/elements/eval_box.dart
index f8285f22cfbec302152d8503a3d3159619e61dab..02cf65bc96150ef50df01ab89f81bd621621c817 100644
--- a/runtime/observatory/lib/src/elements/eval_box.dart
+++ b/runtime/observatory/lib/src/elements/eval_box.dart
@@ -42,7 +42,10 @@ class EvalBoxElement extends ObservatoryElement {
       map['expr'] = expr;
       results.insert(0, map);
       callback(expr).then((result) {
-          map['value'] = result;
+        map['value'] = result;
+      }).catchError((e, st) {
+        map['error'] = e.message;
+        app.handleException(e, st);
       });
     }
   }