Ignore requests to trigger or set active tasks with flow=none

[hjoliver]

Close #5635

What: see title

Why: triggering (or setting) with --flow=none should by definition have no impact on current or future flows.

(Tasks in n=0 have flows assigned already, and only cylc remove can undo that).

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• Changelog entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[oliver-sanders]

Sadly, we can't get the command to fail validation (would requires memory access from the server thread, nasty), so I think this is the best handling of the situation we are going to be able to come up with.

Given that we're ignoring the trigger completely, ERROR might make more sense than WARNING.

[hjoliver]

Agreed, I've upgraded the warnings to errors.

[dwsutherland]

Agreed, I've upgraded the warnings to errors.

Just need to upgrade the test from warning to error it seems.

LGTM otherwise 👍