Improve monitoring for payment notification processor

Errors like bug #1030019 can cause the payment notification processor to start failing, but keep writing (tracebacks) to the log files, so the Nagios alerts we have in place never raise a warning.

We should improve our monitoring in a way that if payments stop getting processed a warning is always sent.
Two current options for this would be:
 - Improve the logfile checking alert so that it ignores lines that contain ERROR, so that it expects to find 'happy' output in the file, not just any output.
 - Add a 'timestamp_entered_state' (or similar) field to the Subscription model, so that we can check for subscriptions that have been in the PaymentNotificationReceived state for more than, say, 10 minutes.