Enforce users to specify a 'pool' dimension when triggering a task.

appengine/swarming/handlers_bot.py

Index: appengine/swarming/handlers_bot.py
diff --git a/appengine/swarming/handlers_bot.py b/appengine/swarming/handlers_bot.py
index ca8259e7f5050dd8bb504240e3575a92e536d56d..ba3dfd7b8134219794722a7f6cbf68b032cbf3d0 100644
--- a/appengine/swarming/handlers_bot.py
+++ b/appengine/swarming/handlers_bot.py
@@ -156,8 +156,8 @@ class _BotBaseHandler(auth.ApiHandler):
     request = self.parse_body()
     version = request.get('version', None)
 
-    dimensions = request.get('dimensions', {})
-    state = request.get('state', {})
+    dimensions = request.get('dimensions') or {}
+    state = request.get('state') or {}
     bot_id = None
     if dimensions.get('id'):
       dimension_id = dimensions['id']
@@ -189,6 +189,9 @@ class _BotBaseHandler(auth.ApiHandler):
       if not bot_id:
         quarantined_msg = 'Missing bot id'
         break
+      if not dimensions.get('pool'):
+        quarantined_msg = 'Missing \'pool\' dimension'
+        break
 
       if not all(
           isinstance(key, unicode) and