Add function to handle loading 2 codebases.

The following lines clutter up the `__init()__` function for DeltaCode: https://github.com/nexB/deltacode/blob/a09514623fc51222230f356a6f911f3e70086cdc/src/deltacode/__init__.py#L61-L70

Create a function to handle this logic, including the various errors that may occur during loading.