Minor clean-up

[em-t]

Minor code clean-up:

• add Raises-section to docstrings where it was missing
• remove unused exception class definitions
• import only relevant exceptions
• rename a handful of test files that were using a different naming convention than most of the test files
• plus other very minor clean-up

[nmaarnio]

I wrote a few single comments but otherwise this is a good clean-up and harmonization, thanks! We started importing exceptions by importing the whole module somewhere along the way, but now I cannot really see any reason to prefer that over the individual imports. So let's do the change you suggested here 👍