Update libarchive error handling; improve error message formatting

tools/libarchivesum.py

@@ -31,4 +31,4 @@ for infile in args.infile:
     try:
         hash_files_in_archive(infile)
     except libarchive.exception.ArchiveError as e:
-        sys.stderr.write(f'Failed to open archive file {infile}: {e}\n')
+        print(f'Failed to open archive file {infile}: {e}\n', file=sys.stderr)