Improve error handling

Nobody has claimed this yet.

Right now we do not catch errors and emit just some generic errors (e.g. ValueError). It would be good to have a custom error class for our exceptions + handle exceptions from other places and turn them into "our" errors.

Some common errors to handle properly:
- errors from geodiff
- file/directory errors (e.g. missing files)
- something is wrong with configuration (wrong table structure / bad content of tables / ...)

The issue names no files, tests, or entry points. Start by locating where ValueError, geodiff errors, file and directory errors, and configuration validation errors are currently handled. Done means defining project-specific exceptions and consistently translating these error categories into them.

Written by the indexing model from the issue text.