pw: Clean up a couple of errx() calls

• Remove a stray backslash.
• Make an error message fit on one line.

No functional change intended.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Sponsored by: Klara, Inc.

(cherry picked from commit bc69d5dffa21cdf228e481b5502fd0f053d65e6d)