JsonApiException namespace issue

[wisepotato]

The current JsonApiException is situated in the JsonApiDotNetCore.Internal namespace, it should be in the JsonApiDotNetCore.Internal.Exceptions, as we have a folder for that.

As discussed in #620, where we discussed that namespaces are a bit of a mess at the moment, we should take some time to correct this.

[bart-degreed]

As part of #714, I have added several custom exceptions that derive from JsonApiException, to help in properly building Error objects that are json:api spec compliant. For example, Title must contain a generic message, while Detail must be case-specific.

I moved JsonApiException and derived classes into a public namespace, so that custom implementations can throw them instead of creating error objects themselves (which is still possible, by the way). I also moved Error and related types into the public Models namespace and added missing fields to make then json:api compliant.