scheduler: log command failures at error level

[oliver-sanders]

Spotted whilst reviewing: #4720

Command errors are logged at the warning level rather than error. These errors aren't CRITICAL (they don't tear down the scheduler), but they are a bit more important than the WARNING level which makes it harder to pick them out in the logs.

Also combine two log entries into one for clarity.

Requirements 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).
• Does not need tests (why?).
• No change log entry required (why? e.g. invisible to users).
• No documentation update required.

[hjoliver]

👍

[hjoliver]

Oops, premature merge. Broke a couple of integration tests on master. Investigating...

... fix #4794