[MNT] Replace Assert Statements with Proper Exceptions

[SimonBlanke]

In the 'runs'-module there are assert statements used for validation. I would rather see exceptions (ValueError, TypeError, etc.) there to have more explicit validation and error messages.

The relevant code can be found here:

• https://github.com/openml/openml-python/blob/main/openml/runs/run.py
• https://github.com/openml/openml-python/blob/main/openml/runs/functions.py

[fkiraly]

Agree, and optimally the most important ones are also listed in the "raises" section of the docstring.