sudo is remakably unhelpful when the target app is missing

sudoers.d contains:

backup ALL=NOPASSWD: /usr/bin/rsync --config=/etc/rsync-backup.conf --server --daemon .

server# su backup
server$ sudo /usr/bin/rsync --config=/etc/rsync-backup.conf --server --daemon .
[sudo] password for backup:

wtf? This setup works on all my other servers, why is it prompting for a password here? Let's check the auth.log

Aug 11 11:35:19 server sudo: pam_unix(sudo:auth): conversation failed
Aug 11 11:35:19 server sudo: pam_unix(sudo:auth): auth could not identify password for [backup]

"conversation failed" means nothing to me, and backup has password-based login disabled, so not having a password is intentional...

After lots of messing around, I *accidentally* stumbled across the fact that /usr/bin/rsync was missing. If it weren't for this accident, I probably never would have figured it out. Can we please have an error message more useful than "conversation failed"? Even now that I know what the problem was, I have no idea how that message was supposed to help me find it...

ProblemType: Bug
DistroRelease: Ubuntu 11.04
Package: sudo 1.7.4p4-5ubuntu7.1
ProcVersionSignature: Ubuntu 3.0.0-4.5-generic 3.0.0-rc6
Uname: Linux 3.0.0-4-generic x86_64
NonfreeKernelModules: nvidia
Architecture: amd64
Date: Thu Aug 11 12:44:46 2011
EcryptfsInUse: Yes
InstallationMedia: Ubuntu 10.10 "Maverick Meerkat" - Release amd64 (20101007)
ProcEnviron:
 LANGUAGE=en_GB:en
 PATH=(custom, user)
 LANG=en_GB.UTF-8
 SHELL=/bin/bash
SourcePackage: sudo
UpgradeStatus: No upgrade log present (probably fresh install)