Fix evil retry loop on poll errors.

appengine/swarming/swarming_bot/bot_code/remote_client_errors.py

Index: appengine/swarming/swarming_bot/bot_code/remote_client_errors.py
diff --git a/appengine/swarming/swarming_bot/bot_code/remote_client_errors.py b/appengine/swarming/swarming_bot/bot_code/remote_client_errors.py
index 964413a69445dbefd1130c73a90c41abc7b7706e..4dc6944d2a3760ea2f3d28fbe1d60c20e5c2bf4d 100644
--- a/appengine/swarming/swarming_bot/bot_code/remote_client_errors.py
+++ b/appengine/swarming/swarming_bot/bot_code/remote_client_errors.py
@@ -20,3 +20,7 @@ class BotCodeError(Exception):
 
 class InternalError(Exception):
   """Raised on unrecoverable errors that abort task with 'internal error'."""
+
+
+class PollError(Exception):
+  """Raised on unrecoverable errors when in RemoteClient.poll."""