M-x ediff-files doesn't notify user of missing ediff-diff-program

[Andrew M. Scott]

Neither

 M-x ediff-buffers buf1 buf2
nor
 M-x ediff-files file1 file2

complain if there isn't an ediff-diff-command on exec-path. Instead, 
a user can type consecutive "n"'s in the *Ediff Control Panel* without
seeing any highlighted diffs (which erroneously leads the user to
think there are no diffs).

This is in contrast to 
M-x diff file1 file2 which *does* properly complain if diff-command is
missing.

Also, why the inconsistency of "diff-command" and "ediff-diff-program"
nomenclature?

Background:

I was trying to diff two versions of my .emacs file (differing by
about 10 lines) on WindowsXP, using this build:
ftp://alpha.gnu.org/pub/gnu/auctex/emacs+auctex-w32-2006-10-10.zip

When neither of these was revealing diffs
 M-x ediff-buffers 
 M-x ediff-files 

I discovered that there was no ediff-diff-command executable
(diff.exe) in my exec-path, as I had removed
C:\Cygwin\bin from my path to resolve an unrelated GAIM dll collision
problem.

Andy Scott