--------------------
Added
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* Handling of exceptions caused by invalid POST data inside views
(the exceptions are suppressed, the client receives a JSON-response
containing the error and some additional info about it).
See updated documentation on views (Reference/easycart.views)
* Additional exception classes for invalid item quantity errors:
- NegativeItemQuantity
- NonConvertibleItemQuantity
- TooLargeItemQuantity
- ZeroItemQuantity