bcachefs: Fix btree node write error message

Error messages should include the error code, when available.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_io.c

@@ -1777,7 +1777,7 @@ static void btree_node_write_work(struct work_struct *work)
 err:
 	set_btree_node_noevict(b);
 	if (!bch2_err_matches(ret, EROFS))
-		bch2_fs_fatal_error(c, "fatal error writing btree node");
+		bch2_fs_fatal_error(c, "fatal error writing btree node: %s", bch2_err_str(ret));
 	goto out;
 }