DataPreprocessor doesn't have it's own failure reason

[jshier]

Alamofire 5 added a DataPreprocessor protocol to enable the easy filtering of Data before processing, mainly to strip various headers in front of JSON. Unfortunately, when the protocol was added, no corresponding ResponseSerializationFailureReason was added, meaning the failure of a DataPreprocessor will be encapsulated in a .customSerializationFailed reason. Luckily, none of the current DataPreprocessors produce errors, so we may never see this issue.

Fixing this will require a major version change due to the addition of an enum case.