dashboard, storage: fix WorkerNotFound on StorageList when DataStore is offline

circle/dashboard/templates/dashboard/storage-list/column-storage-used_percent.html

+{% load i18n %}
+{% with record.used_percent as used_percent %}
+{% if used_percent < 0 %}
+<div>
+  {% trans 'The DataStore is offline.' %}
+</div>
+{% else %}
 <div class="progress">
-  {% with record.used_percent as used_percent %}
   <div class="progress-bar progress-bar-stripped
     {% if used_percent < 80 %}progress-bar-success
     {% elif used_percent < 90 %}progress-bar-warning
@@ -7,5 +13,6 @@
     role="progressbar" style="min-width: 30px; width: {{ used_percent }}%">
     {{ used_percent }}%
   </div>
-  {% endwith %}
 </div>
+{% endif %}
+{% endwith %}

circle/storage/models.py

@@ -119,9 +119,11 @@ class DataStore(Model):
 
     @property
     def used_percent(self):
-        stats = self.get_statistics()
+        try:
+            stats = self.get_statistics()
+        except WorkerNotFound:
+            return -1 # invalid value for unreachable
         free_percent = float(stats['free_percent'])
-
         return int(100 - free_percent)
 
     @property