Enforce defining a content directory when the prerecorded option is used

[jvdrhoof]

If the prerecorded option is set for LocalUser.PCSelfConfig in config.json, a content directory should be defined through PrerecordedReaderConfig.folder. If the content directory is not set, an error should be returned instead of assigning a directory that not everyone will have by default (currently hidden in Unity assets rather than in a config file). If the content directory does not exist, an error should be returned as well.