macOS: Launching prints 'Unload failed: 5: Input/output error'

[maleadt]

This error isn't problematic, and only happens because we (to be safe, IIUC) try to unload the service multiple times:

`launchctl unload -w $(plist_path)` = `launchctl unload -w /Users/julia/Library/LaunchAgents/org.julialang.buildkite.default-honeycrisp-HL2F7YQ3XH.0.plist`
`launchctl unload -w $(plist_path)` = `launchctl unload -w /Users/julia/Library/LaunchAgents/org.julialang.buildkite.default-honeycrisp-HL2F7YQ3XH.0.plist`
Unload failed: 5: Input/output error
Try running `launchctl bootout` as root for richer errors.
`launchctl load -w $(plist_path)` = `launchctl load -w /Users/julia/Library/LaunchAgents/org.julialang.buildkite.default-honeycrisp-HL2F7YQ3XH.0.plist`

Creating an issue so that we either document or fix this; I wasn't sure whether this was an issue when debugging other problems.