Exception handler fails when multiple errors occur at the same time

[Senjai]

Per this line https://github.com/solidusio-contrib/solidus_active_shipping/blob/master/app/models/spree/calculator/shipping/active_shipping/base.rb#L136

This isn't always a hash, there may be an array of errors here, and thus the handler fails when trying to call key? on the array. We should either add further type checking, or use Array.wrap here.