Replace and rewrite "assert" statements in non-test code

See https://www.reddit.com/r/learnpython/comments/zd217y/is_using_assert_considered_good_practice/

> assert statements should only be used for debugging purposes. They are removed when not running in debug mode (i.e. when invoking the Python command with the -O or -OO options).

> Instead use a raise statement to throw a proper, informative runtime exception, if you need to. That would definitely be best practice when there'd otherwise be no sensible return value for the function.

We are using a few asserts in our main code. See https://github.com/search?q=org%3Aaboutcode-org+NOT+path%3A*test*++"assert"++language%3APython&type=code