Xtrigger validation: log traceback for unexpected user validation errors

[MetRonnie]

Small change with no associated issue

Can test by installing a custom xtrigger module with a validate function that raises a RuntimeError, for example

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).
• No dependency changes
• Tests are not needed
• Changelog entry not needed as minor
• 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]

(LGTM, haven't tested)