Improve error logging for contact file checks.

[wxtim]

Small change to improve the logging of workflow_files checking for contact files.

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).
• Logging changes only: No tests, changelog, documentation &c

[oliver-sanders]

@wxtim if you want this in 8.3.1, make sure it goes on the 8.3.x branch when it is created.

[MetRonnie]

Just create the 8.3.x branch on/after this commit? Edit: someone already created it, I have fast-forwarded it to the merge commit of this PR