Log command validation errors

[oliver-sanders]

Follow up from #6112

Full exception traceback is reported in debug mode, but nothing gets logged in normal mode which is a tad unhelpful for determining why user's commands failed in retrospect, whoops.

To test, try using this cylc set command:

$ cylc vip
$ cylc set <workflow>//1/a --pre=1/a,all
--pre=all must be used alone

The error that is returned to the user via the CLI should also be logged in the workflow log like so:

$ cylc cat generic
...
INFO - Pausing the workflow: Paused on start up
WARNING - Command "set" received.
    InputError: --pre=all must be used alone

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).
• CHANGES.md 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.

[wxtim]

Looks reasonable. Tested as working.