ionice of util-linux-ng returns exit code 0 when complaining about it's usage

Issue

• Calling ionice that is part of the util-linux-ng package with bad arguments, terminates with exit code of 0, generally understood to indicate "no problem" but without really running the command specified.

• This is e.g. bad when used in a pre-backup script as it will not have the backup abort/report an issue in that case.

# ionice -c idle some-command; echo $?
ionice: cannot parse number 'idle'
0

• Expected to return something != 0.