Problem/Motivation
If a row fails to migrate then the debug message is:"Successful migration of $source_id to $destination as part of the {$migration->id()} migration. The source row status is " . $row['source_row_status']
e.g.
Successful migration of a:3:{s:9:"sourceid1";s:5:"forum";s:9:"sourceid2";s:1:"0";s:9:"sourceid3";s:7:"garland";} to a:0:{} as part of the d6_block migration. The source row status is 3
Proposed resolution
s/Successful/Failed
would make much more sense.
Remaining tasks
Adjust the failure error mesage.