The alert is the stock one created when configuring log shipping as follows:-

EXEC msdb.dbo.sp_add_alert @name=N'Log shipping Primary Server Alert.',

@message_id=14420,

@severity=0,

@enabled=1,

@delay_between_responses=0,

@include_event_description_in=1,

@category_name=N'[Uncategorized]',

@job_id=N'00000000-0000-0000-0000-000000000000'

The job successfully raises the error, and this is logged in the SQL Server log.

If I manually raise the error, then it still doesn't fire the trigger.