Update `WordPressOrgNetwork` to return `NetworkError` similar to `AlamofireNetwork`

[jaclync]

From the original discussion #11558 (comment), the network requests from AlamofireNetwork are now returning NetworkError after #11110 while WordPressOrgNetwork is still returning AFError. It can be confusing in the app where we have two ways of error handling based on the status code based on the network implementation. It'd be more consistent if all Network implementations return the same error type NetworkError based on the status code, also to abstract the implementation of the networking library (Alamofire).

[peril-woocommerce]

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