Add failed_once category to Sheriff-O-Matic.

Tools/GardeningServer/ui/ct-unexpected-failures.html

Index: Tools/GardeningServer/ui/ct-unexpected-failures.html
diff --git a/Tools/GardeningServer/ui/ct-unexpected-failures.html b/Tools/GardeningServer/ui/ct-unexpected-failures.html
index 4bb6946c82a6218acb357356cd2adbcfcc71e3a5..2bfb94f93832f42f9b562b299a8ff86e856c17f9 100644
--- a/Tools/GardeningServer/ui/ct-unexpected-failures.html
+++ b/Tools/GardeningServer/ui/ct-unexpected-failures.html
@@ -43,6 +43,9 @@ found in the LICENSE file.
     <ct-tree-status status="{{ treeStatuses['blink'] }}" state="{{ treeStatuses['blink'].status }}"></ct-tree-status>
     <ct-party-time failures="{{ failures.failures[tree].length == 0 }}"></ct-party-time>
     <ct-failure-stream category="default" groups="{{ failures.failures[tree] }}" commitLog="{{ commitLog }}" tree="{{ tree }}"></ct-failure-stream>
+    Failures that have only happened once (on one bot):
+    <ct-failure-stream category="failed_once" groups="{{ failures.failures[tree] }}" commitLog="{{ commitLog }}" tree="{{ tree }}"></ct-failure-stream>
+    Snoozed failures:
     <ct-failure-stream category="snoozed" groups="{{ failures.failures[tree] }}" commitLog="{{ commitLog }}" tree="{{ tree }}"></ct-failure-stream>
   </template>
   <script>