Log any cat-log failure when listing log files

[MetRonnie]

When trying to set up cylc hub, Dave encountered cat-log problems which were swallowed by the UIS. This PR ensures any such errors are logged.

Check List

• [x] I have read CONTRIBUTING.md and added my name as a Code Contributor.
• [x] Contains logically grouped changes (else tidy your branch by rebase).
• [x] Does not contain off-topic changes (use other PRs for other changes).
• [x] Tests are not needed
• [x] No changelog entry needed as minor
• [x] No docs needed
• [x] If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[MetRonnie]

I'm inclined to say no automated tests are needed for this

[MetRonnie]

I realised that there are a few ways cat-log will exit non-zero when navigating in the UI (e.g. navigating to /log/non-exist), so have settled for just logging the stderr and returning an empty list as before