Migrator shows wrong migration as missing if unfulfillable

If a migration's dependency is considered unfulfillable, the system reports the dependency as missing instead of the actual missing migration. This missing migration is often farther down the dependency chain.

Steps to reproduce:

1. Rename phpbb/db/migration/data/v310/bot_update.php to bot_update2.php
2. Rename that migration's class to bot_update2
3. Run the migrations
4. Receive error:

   The migration "\phpbb\db\migration\data\v31x\v319rc1" is not fulfillable, missing migration "\phpbb\db\migration\data\v31x\v318".

Expected result:
The bot_update migration is reported as missing, not the first dependency.