Fixing `LoadJetsFromFile` bug and extend error message for `GetRejection`

This MR fixes two things:

• When loading the validation/test files, there were an issue that when you request 2M jets and 5M are in the file, you will only get 2M and then the cutting is applied. So you will end up with less jets than you want although there are still jets left. This is fixed now.
• Testing the fix via unit tests is provided.
• Adding a more detailed error message when in GetRejection on class has not enough jets for a to tight working point (the ZeroDivisionError. This should help finding the issue.
• All plots: Updated due to the fix of the loading of the validation/test files. Pipeline is running smooth.