Assert on ValueError exception messages

[Andrew Hawker]

There are many cases where a ValueError can be raised by any number of functions across most of the modules in this package.

I am relatively confident that all of the @pytest.raises(ValueError) calls are correct based on code coverage metrics. However, I was proven wrong today and had to address some of them with #61.

The scope of this task is to go through all tests that use @pytest.raises, capture the exception and perform an additional assertion of the exception message to confirm that we're hitting the exact code path expected.