Must record tape only when the query for workflows succeed

[kinow]

The query for workflows may take a while, as it does not trigger a cylc scan, but instead does a look up on a dictionary populated every N seconds by a Tornado PeriodicCallback.

Once that query succeeds, then we can proceed with the rest of the script.

[kinow]

Quite hard, as you need to query within a with vcr... context manager, which automatically creates a cassette. Only if it's possible to abort the recording of a cassette...