Output usage errors to ErrWriter

[kke]

What type of PR is this?

• bug

What this PR does / why we need it:

Usage errors were being output to stdout (command.Writer) instead of stderr (command.ErrWriter).

Which issue(s) this PR fixes:

Release Notes

(REQUIRED)

Usage errors now go to the stderr stream. To capture them, use `command 2>&1 > foo.txt`.
If you have used something like `Writer: io.Discard` to suppress errors, you now need to suppress
them also for `ErrWriter`.

[meatballhat]

Thank you!