LP #1549301: buf_dblwr_init_or_load_pages now returns an error code, but caller not updated

**Reported in Launchpad by Laurynas Biveinis last update 08-03-2016 12:45:52

Copy of http://bugs.mysql.com/bug.php?id=80496:

[24 Feb 14:00] Laurynas Biveinis
Description:
This is a minor code review type of bug:

buf_dblwr_init_or_load_pages, previously a void function, now returns a dberr_t since [1]. But its sole caller does not bother to check it.

[1]:

commit 98909cefbc37e54efc6452c7e95bccbf64ac9213
Author: Sunny Bains <Sunny.Bains@oracle.com>
Date: Wed Mar 4 13:58:59 2015 +1100

WL#7696 - Backport code from mysql-trunk to mysql-5.7

How to repeat:
Grep for buf_dblwr_init_or_load_pages

Suggested fix:
Check the return code in SysTablespace::read_lsn_and_check_flags, add attribute warn_unused_result.