dry-run: Base table or view not found: 1146 Table 'migrate_map_action_settings' doesn't exist

The problem is that if you run the command like this:
drush migrate:orphans-purge --dry-run my_migration_item
.. it thinks that my_migration_item is an argument passed to the --dry-run option.

It works if you run the command this way:
drush migrate:orphans-purge my_migration_item --dry-run

This makes the examples more clear, and forces the dry-run option to be boolean.

Awesome contribution! Thanks!