diff should explain on stderr when exiting 2 due to differing binary files

[Matt McCutchen]

Currently, when diff exits 2 due to differing binary files, it prints
nothing to stderr.  Thus, when I ran into this error a few days ago, I
had no idea what the problem was.  I eventually noticed the "Binary
files ... differ" buried in the output to stdout, but I wasn't aware
that this was considered an error.  After several hours of
troubleshooting, I finally ran across the mention of exiting 2 on
differing binary files in the info documentation.

I think diff should save users like me time and frustration by
printing a message to stderr explaining the error.  Here's one
possibility for the message:

diff: output does not capture differences between binary files foo and bar

Matt