* Add new exceptions for out of stock scenarios
These are needed to differentiate between the different stock validation errors, and so we can create the correct error message.
* Catch new out of stock exceptions when checking the cart for errors
This is so we can get the cart sent back to the client, if we don't catch these, then the route will just return a 500 error and crash.
* Add ArrayUtils class
This will contain methods used to operate on arrays that don't fit anywhere else.
* Handle the case in Checkout where the error is already a WP_Error
This will happen when the cart fails validation.
* Handle StockAvailabilityException in AbstractRoute
This will happen when an item or number of items in the cart are out of stock/insufficient stock.
* Throw exceptions for each type of invalid stock in validate_cart_items
This will allow us to create an error message for each type of violation to display to the user.
* Display additional error notices returned by the API
* Fix wording when throwing exceptions relating to stock
* Handle TooManyInCartException in CartController
* Abstract the merging of cart, status, and additional data into new fn
This allows us to simplify the way errors are returned from the API. The reason we have to add all of the data at once is because of how WP_Error works with the additional data, if there is already existing data in a WP_Error object, it gets moved into additional_data. By adding all of the data in one place, we stop this from happening. Also since we're only adding status and/or cart explicitly, it makes sense to just do it in one place.
* Add get_route_error_response_from_object method
This is so we can differentiate between a string and WP_Error object.
* Remove unnecessary slashes from WP_Error instantiation
* Add option to enclose each item in quotes in natural_language_join
* Abstract adding error messages to error object into single function
A lot of code was repeated, so doing this cuts down on that and ensures any changes only need to be made in one place.
* Create new parent exception for each type of out of stock exception
This is so we don't have to repeat code inside each different exception and we can simply inherit StockAvailabilityException.
* Catch the generic StockAvailabilityException in get_cart_item_errors
* No longer recalculate totals in validate function
It is not needed, the totals are recalculated elsewhere. This call was superfluous.
* Reduce nesting, and only throw exception if error object has errors
* Improve comment on get_route_error_response_from_object method
* Fix nesting when throwing the InvalidStockLevelsInCartException
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