broadcast: report invalid settings

[oliver-sanders]

• We already report invalid cycle points and namespaces.
• However, we have been relying on client-side validation for settings (which doesn't apply to the GraphQL mutation).
• This also raises the potential for inter-version compatibility issues going unreported.
• This commit explicitly handles invalid settings in the same way as invalid cycle points and namespaces so that they are reported back to the user.
• Closes the issue part of command validation: broadcasts #6429.

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.