Ensure callback chain is aborted when store credit is not valid

This is needed to avoid discard to set deleted_at anyway.

Now that we are not running this method as a validation anymore we need
to stop the callback chain by throwing abort, see:

https://api.rubyonrails.org/classes/ActiveModel/Callbacks.html

core/app/models/spree/store_credit.rb

@@ -268,6 +268,7 @@ def validate_category_unchanged
  def validate_no_amount_used
  if amount_used > 0
  errors.add(:amount_used, 'is greater than zero. Can not delete store credit')
+ throw :abort
  end
  end