Writing Errorlog To Sql Errorlog Problem

Hello all,

I don't know what I am doing wrong.
In my environment, we have monitoring tool that scan through sql errorlog
every 10minutes to capture some error that occurs.
I have been trying for days now to create an alert for any of our failed
backup to appear in the sql errorlog with the code below. After I add
message to the sysmessage table with sp_addmessage, and raising the error
with log option, I modified the existing scheduled backup job by creating
an alert with the num in the code.
For some reason, this error will not just show up in the sql errorlog when
it failed.
Has anyone any advise on what to do?

The code is:

sp_addmessage 50001, 25, 'Backup Failed for the '%.*ls' database.Please Contact MSSQL DBA Immediately!!!! '
GO
DECLARE @DBID INT
SET @DBID = DB_ID()
DECLARE @DBNAME NVARCHAR(128)
SET @DBNAME = DB_NAME()
RAISERROR (50001, 25, 1, @DBID, @DBNAME) with log
GO
Thanks
SA