The name `ActiveModel::AttributeAssignment::UnknownAttributeError` is
too implementation specific so let's move the constant directly under
the ActiveModel namespace.

Also since this constant used to be under the ActiveRecord namespace, to
make the upgrade path easier, let's avoid raising the former constant
when we deal with this error on the Active Record side.

Minor style changes across the board. Changed an alias to an explicit
method declaration, since the alias will not be documented otherwise.

Allows to use it for any object as an includable module.