Fix changeset errors

[kelostrada]

This fixes error handing, example:

There are two changes. First of all we had an issue that this was not working at all and throwing like this:

(CaseClauseError no case clause matching: {:error, {:error, #Ecto.Changeset<action: :update, changes: %{is_admin: "**redacted**"}, errors: [is_admin: {"cannot set is_admin for non official emails", []}], data: #Api.Accounts.User<>, valid?: false>}})

This happens because the error handling in lib/kaffy/resource_callbacks.ex wraps the "unexpected" errors in the error tuple, which is pointless when the error is already an error tuple. For example this results in {:error, {:error, Ecto.Changeset.t()}} tuple which can't be then handled correctly.

Second after I fixed this I noticed the errors are not displayed in details so this was the second part of the fix. You can notice I added a simple helper function - add_class/2 which basically appends the class in the opts instead of overwriting it every time. We have even places where we put the new class one after another which makes the old value redundant.

[ghenry]

Thanks for taking the time to submit this PR! Very kind of you.