pwd: Error out if writing to stdout failed

POSIX requires us to print a diagnostic and return a non-zero exit
code if writing to stdout failed.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D55227

(cherry picked from commit 5b398611607b0dab2f2550ef73f62d41dab6fac5)