Stop workflow on failed task when debugging

### What problem does your feature request solve?

Its annoying how the workflow carries on after a job fails, as it means you then need to go and chase down the log files after the task disappears from the GUI.

On the other hand, when running CSET as a user, it is nice to still have some output even if just a few diagnostics fail.

### Describe the solution you'd like

If logging is set to debug, we should require the task to succeed, else we should continue.

### Describe alternatives you've considered

Start by locating the workflow task failure handling and the code that reads the logging level; no specific files or tests are named in the issue. Confirm that debug logging stops the workflow on a failed task while normal logging continues, including the diagnostic-failure case described in the request.

Written by the indexing model from the issue text.