#3630664 --feedback on migrate:rollback prints the import message with zero counts

David Segesvárirequested to merge
issue/migrate_tools-3630664:3630664---feedback-on-migraterollback into 6.1.x

Restores rollbackMessage(FALSE) in MigrateExecutable::onPostRowDelete(); it was replaced by progressMessageEmit(FALSE) in #3067165, so rollback feedback printed the import message with zero counts. Adds DrushCommandsTest::testRollbackFeedback(), which fails without the fix.

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