cylc clean: don't bail early on a list of workflows

[hjoliver]

cylc clean shouldn't bail out if it runs into a problem cleaning one workflow in a list (i.e. it should continue on to clean the next ones).

E.g., install and run a workflow that uses a non-existent job platform, then try to clean it and some others using a wildcard.

[hjoliver]

See also #4673

[oliver-sanders]

I think we can live with this (provided the non-existant job platform issue can be handled), have downgraded the "bug" label, can we bump this to 8.x?

[hjoliver]

If I ask to clean 3 workflows and it stops at 1, that's a bug innit.

I was expecting this would be a trivial fix, but if not, we can bump it. I'll have a look...

[hjoliver]

BTW, I've actually found this one pretty annoying lately. Any functional test that fails and used a fake platform - e.g. the "intelligent host selection" tests broken initially by #4938) - prevents other tests from being cleaned up with cylc clean cylctb* because of this (and #4935).