Cylc Review: Check for `runN`

[wxtim]

Description

If I put a name of a non-existant workflow I get traceback with a helpful error message.

If I put the name of a workflow that exists, but don't include the runN section I get unhelpful traceback.

At Cylc 8 if I put in a workflow name without the runN into this dialog that should be automatically checked and added.