diff: Report I/O errors in Stone algorithm

In the legacy Stone algorithm, we do a first pass over the files to
check if they're identical before we start diffing them. That code
would correctly set the exit status if an I/O error was encountered,
but would not emit an error message. Do so.

PR: 292198
MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: thj
Differential Revision: https://reviews.freebsd.org/D55125

(cherry picked from commit f8c12e6e3874cdd353fb16785da6f4e7eb134cd9)