[ticket/11266] Better message for when composer dependencies are missing.

PHPBB3-11266
Merge pull request #2512 from bantu/ticket/11266

[ticket/11266] Better message for when composer dependencies are missing...

* bantu/ticket/11266:
  [ticket/11266] Better message for when composer dependencies are missing.
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/11266] Better message for when composer dependencies are missing.