TwoColumnOperatorTransformer bugs

[TommyMatthews]

Input check for columns needs fixing as it currently

• Does not raise a value error when instantiated with empty columns list
• Does not raise a type error when columns instantiated with a string

[ChaitanMohr]

An existing check grouped these two issues together with an and statement, so only raised a ValueError when both issues were happening. Split that up into two separate checks instead to raise a ValueError or TypeError.

[ChaitanMohr]

Also written 2 unit tests to check these distinct potential errors