deserialize error messages directly from error hash instead of an array of messages

Currently errors are interpreted and received in the following format:

    <?xml version="1.0" encoding="UTF-8"?><errors><error>Age can't be blank</error><error>Name can't be blank</error><error>Name must start with a letter</error><error>Person quota full for today.</error></errors>
    {"errors":["Age can't be blank","Name can't be blank","Name must start with a letter","Person quota full for today."]}

With the attached patch the following will be correctly interpreted:

    <?xml version="1.0" encoding="UTF-8"?><errors><age type="array"><age>can't be blank</age></age><name type="array"><name>can't be blank</name><name>must start with a letter</name></name><base type="array"><base>Person quota full for today.</base></base></errors>
    {"errors":{"age":"can't be blank","name":["can't be blank","must start with a letter"],"base":"Person quota full for today."}}

We feel that this is a more flexible and direct solution; the code to interpret this is more readable and shorter and it allows for such things as translating error messages in the backend and being able to still associate them with the correct attribute in the frontend.

One thing that needs fixing before applying this is #5615 Support multiple errors per attribute in ActiveModel Errors JSON serialization, which will then allow to have multiple error messages on one attribute.