As suggested by jilles in https://reviews.freebsd.org/D17146:
> This is not wrong, but perhaps some details about different types
> of errors may be helpful, such as that nonexistent usernames in -U
> are immediately fatal while an empty result (such as from only
> nonexistent process IDs in -p or only existing users that do not
> currently run any process in -U) still writes the header line (if
> not disabled) and also returns a non-zero exit status.