Fix cargo xtask fmt --check

[nicholasbishop]

Format checking was printing errors but not exiting non-zero, so the CI didn't actually catch formatting errors since da55df6.

Fix that, and fix up a few format errors that snuck in.

Checklist

• Sensible git history (for example, squash "typo" or "fix" commits). See the Rewriting History guide for help.
• Update the changelog (if necessary)