`__getitem__` should raise KeyError if the key is not found

[demosdemon]

config-reader-python/platformshconfig/config.py

Line 429 in 7084bcd

return self._environmentVariables.get(check_name)

[Crell]

Alternatively we can refactor getitem out completely and just use @Property methods, like we do in Node.Js. But then the error handling should still be made consistent to raise an exception on a missing value, not return None.