Raise exception if dividing by 0

[ericsaupe]

While this should never happen it is possible to call this method
and subsequently try to save a NaN value if the line_item has
already been fully cancelled.

Description

This came up in our store today. I was unable to recreate it but somehow one of our customer service representatives short shipped an item and the adjustment, and subsequent adjustment total on the order, went to NaN. Looking into the calculation it looks like it may be possible for the short ship compute amount to return that result. This PR raises an exception if, somehow, that method is called and tries to divide by 0.

Checklist:

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message
• I have updated Guides and README accordingly to this change (if needed)
• I have added tests to cover this change (if needed)

[jacobherrington]

👍 Probably not a bad idea. Thanks!

[kennyadsl]

I'm fine with this change but I'd move the raise in the caller method. Let me know your thoughts and thanks!

[kennyadsl]

@ericsaupe just thinking: wouldn't this lead to another exception ZeroDivisionError without the need to explicitly raise it?

[ericsaupe]

@kennyadsl because quantity_of_line_item is a BigDecimal it actually returns BigDecimal('-Infinity') when quantity_of_line_item is BigDecimal(0)

[ericsaupe]

It probably makes more sense to really return that error. I'll change that.

[kennyadsl]

Thanks @ericsaupe!

[tvdeyen]

Looks fine