Improve invalid nested value error message

[tfausak]

This pull request is in response to #255. When you define a hash with default: {}, we eagerly build the default value. If one of the inputs inside that hash doesn't have a default, you'll get an error. For example:

class Example < ActiveInteraction::Base
  hash :x, default: {} do
    string :y
  end
end
# ActiveInteraction::InvalidNestedValueError: ActiveInteraction::InvalidNestedValueError

Since there's no error message, it's basically useless. This pull request adds a message to that error. Now it looks like this:

class Example < ActiveInteraction::Base
  hash :x, default: {} do
    string :y
  end
end
# ActiveInteraction::InvalidNestedValueError: y: nil

Can you review this, @AaronLasseigne?

[AaronLasseigne]