`CacheManager::resolve` is currently allowed to return `false` if the resolver is invalid, see https://github.com/liip/LiipImagineBundle/pull/301/files#diff-34e30bd50b4377934c8d364789afc96cR196

I'm not a huge fan of it at all and would rather remove the validation from the `CacheManager::resolve` method completely. If the user is requesting a missing filter, it should not be silently ignored, but throw an `\OutOfBoundsException` (`CacheManager::getResolver` should throw it directly).