diff: Print an error message on I/O error

If an error occurs while atomizing either file, immediately print an
error message using the error code returned by the atomizer, and set
the error bit in the exit status.

PR: 292198
MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: thj, kevans
Differential Revision: https://reviews.freebsd.org/D55109