Give better error message for invalid nested defaults

[tfausak]

This pull request changes the error message when you have an invalid nested default. Its goal is to make it clearer what the problem is. Issue #309 may have been avoided if something like this was in place.

Here is an example of the new message:

class Example < ActiveInteraction::Base
  hash :h,
    default: {} do
      boolean :b
    end
end
# ActiveInteraction::InvalidDefaultError: h: {} (b: nil)

[tfausak]

The wording here could be a lot better.

[AaronLasseigne]

Looks good to me.

[tfausak]

Great! @elfassy, how do you feel about this pull request? Would it have helped you understand what was going on with hash defaults?

[elfassy]

👍