grep.c: use error_errno()

While at there, improve the error message a bit (what operation failed?)

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

grep.c

@@ -1732,7 +1732,7 @@ static int grep_source_load_file(struct grep_source *gs)
 	if (lstat(filename, &st) < 0) {
 	err_ret:
 		if (errno != ENOENT)
-			error(_("'%s': %s"), filename, strerror(errno));
+			error_errno(_("failed to stat '%s'"), filename);
 		return -1;
 	}
 	if (!S_ISREG(st.st_mode))
@@ -1743,7 +1743,7 @@ static int grep_source_load_file(struct grep_source *gs)
 		goto err_ret;
 	data = xmallocz(size);
 	if (st.st_size != read_in_full(i, data, size)) {
-		error(_("'%s': short read %s"), filename, strerror(errno));
+		error_errno(_("'%s': short read"), filename);
 		close(i);
 		free(data);
 		return -1;