node list: Fix page load error when no task result

The page now loads when the node can't reach the monitor info values e.g. cpu percent. It gets a default value 0. And if a node's monitor info unreachable shows
an error message.

circle/dashboard/views/node.py

@@ -210,9 +210,15 @@ class NodeList(LoginRequiredMixin, GraphMixin, SingleTableView):
             return super(NodeList, self).get(*args, **kwargs)
 
     def get_queryset(self):
+        self.wrong_node_message()
         return Node.objects.annotate(
             number_of_VMs=Count('instance_set')).select_related('host')
 
+    def wrong_node_message(self):
+        for node in Node.objects.all():
+            if node.monitor_info is None:
+                messages.error(self.request, "Can't reach "+node.name+" monitor info")
+
 
 class NodeCreate(LoginRequiredMixin, SuperuserRequiredMixin, TemplateView):
 

circle/vm/models/node.py

@@ -34,8 +34,9 @@ from django.db.models import (
 )
 from django.utils import timezone
 from django.utils.translation import ugettext_lazy as _
+from django.contrib import messages
 
-from celery.exceptions import TimeoutError
+from celery.exceptions import TimeoutError, TaskRevokedError
 from model_utils.models import TimeStampedModel
 from taggit.managers import TaggableManager
 
@@ -315,7 +316,7 @@ class Node(OperatedMixin, TimeStampedModel):
                 queue=self.get_remote_queue_name('vm', priority),
                 expires=timeout + 60)
             return r.get(timeout=timeout)
-        except (TimeoutError, WorkerNotFound):
+        except (TimeoutError, WorkerNotFound, TaskRevokedError):
             if raise_:
                 raise
             else:
@@ -363,21 +364,23 @@ class Node(OperatedMixin, TimeStampedModel):
     def driver_version(self):
         return self.info.get('driver_version')
 
+    def get_monitor_info(self, metric):
+        if self.monitor_info is None:
+            return 0
+        elif self.monitor_info.get(metric) is None:
+            return 0
+        else:
+            return self.monitor_info.get(metric)
+
     @property
     @node_available
     def cpu_usage(self):
-        if self.monitor_info.get('cpu.percent') is not None:
-            return self.monitor_info.get('cpu.percent') / 100
-        else:
-            return 0
+        return self.get_monitor_info('cpu.percent') / 100
 
     @property
     @node_available
     def ram_usage(self):
-        if self.monitor_info.get('memory.usage') is not None:
-            return self.monitor_info.get('memory.usage') / 100
-        else:
-            return 0
+        return self.get_monitor_info('memory.usage') / 100
 
     @property
     @node_available