File Abstraction Layer

We currently have a lot of different exceptions - which is good from a semantic point of view. This hierarchy should however be reviewed to get a clear picture of what different kinds of extensions we have and how they relate - and when to use which. This will also help us in identifying which exceptions might be missing.

This has to be done before 4.7 because we will need to properly document it for developers and the hierarchy should be rather fixed when releasing FAL to the public.