Rework error handling in the service protocol and in Observatory.

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

Index: runtime/observatory/lib/src/elements/vm_connect.dart
diff --git a/runtime/observatory/lib/src/elements/vm_connect.dart b/runtime/observatory/lib/src/elements/vm_connect.dart
index e9b8f55b23c95b61dccb22ea92b3daca23ebcf8d..5d42edc4e8a6ad17a1bfd1cb6ee49e21654b4214 100644
--- a/runtime/observatory/lib/src/elements/vm_connect.dart
+++ b/runtime/observatory/lib/src/elements/vm_connect.dart
@@ -39,7 +39,9 @@ class VMConnectTargetElement extends ObservatoryElement {
     }
     event.preventDefault();
     WebSocketVM currentVM = app.vm;
-    if ((currentVM == null) || (currentVM.target != target)) {
+    if ((currentVM == null) ||
+        currentVM.isDisconnected ||
+        (currentVM.target != target)) {
       _connectToVM(app, target);
     }
     var href = node.attributes['href'];