correctly print errors for active_model_errors for model without a table

[sahglie]

If you create a ActiveModel class that does not have a table behind it, ap will throw an exception when trying to print the objects errors:

class TableFreeModel
    include ::ActiveModel::Validations
    attr_reader(:name)

    def attributes
      {"name" => name}
    end
end

model = TableFreeModel.new
model.errors.add(:name, "can't be blank")
ap model.errors

#=> NoMethodError (undefined method `column_names' for #<Class:0x00007fe3b6c833f8>)

This pull request fixes this error so that errors are correctly printed regardless of whether the model is back by a table

Fixes #34
Fixes #40

[HarlemSquirrel]

Simply amazing. Fixes two issues in one pull. 🤘

[dmitryzuev]

Oh, this is magic, I just stumbled upon this error when trying to display model decorated by draper gem, and it was merged into master two hours ago!

Sweet 👍

[manuelmeurer]

Brilliant, thanks! Could we get a new patch release? 😸

[HarlemSquirrel]

Done.
https://github.com/amazing-print/amazing_print/releases/tag/v1.2.1