Make vipw error message less cryptic

Make vipw error message less cryptic

Unable to find an editor, vipw would give this error:
# env EDITOR=fnord vipw
vipw: pw_edit(): No such file or directory

vigr or crontab do better:
# env EDITOR=fnord crontab -e
crontab: no crontab for root - using an empty one
crontab: fnord: No such file or directory
crontab: "fnord" exited with status 1

After this change, vipw behaves more like vigr or crontab:
# env EDITOR=fnord vipw
vipw: fnord: No such file or directory
vipw: "fnord" exited with status 1

Reviewed by:	rpokala, emaste
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D25369