[ticket/13064] Validate the migrations provided to migrator::set_migrations()

PHPBB3-13064
Merge pull request #2951 from Nicofuma/ticket/13064

[ticket/13064] Validate the migrations provided to migrator::set_migrations()

* Nicofuma/ticket/13064:
  [ticket/13064] Validate the migrations provided to migrator::set_migrations()