Catch missing Parler-related settings at application load time

Catch missing Parler-related settings at application load time: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). When integrating Shuup into a Django project, it's quite easy to set up the obvious bits (INSTALLED APPS, the database, etc.) but forget about the…