Improve assert error msg

[Bachibouzouk]

Addresses part of #540

Changes proposed in this pull request:

• Write explicit test failure messages hinting to the developer what is the meta cause of the test failure

The following steps were realized, as well (if applies):

• Write docstrings to your code
• Update the CHANGELOG.md
• Apply black (black . --exclude docs/)
• Check if benchmark tests pass locally (EXECUTE_TESTS_ON=master pytest)

_{For more information on how to contribute check the CONTRIBUTING.md.}

[Bachibouzouk]

@smartie2076 you can still look at the PR and let me know if there is something which need change