Fix error_description on successful response

[aiperon]

When request returns with a successful status, response doesn't contain Error element.
As a result, error_description throws an exception. It's confusing.

Steps to reproduce

require 'ups'
server = UPS::Connection.new(test_mode: true)
response = server.rates do |rate_builder|
  rate_builder.add_access_request 'API_KEY', 'USERNAME', 'PASSWORD'
  rate_builder.add_shipper company_name: 'Veeqo Limited', phone_number: '01792 123456', attention_name: 'Veeqo', address_line_1: '11 Wind Street', city: 'Swansea', state: 'Wales', postal_code: 'SA1 1DA', country: 'GB', shipper_number: 'ACCOUNT_NUMBER'
  rate_builder.add_ship_from company_name: 'Veeqo Limited', phone_number: '01792 123456', attention_name: 'Veeqo', address_line_1: '11 Wind Street', city: 'Swansea', state: 'Wales', postal_code: 'SA1 1DA', country: 'GB', shipper_number: 'ACCOUNT_NUMBER'
  rate_builder.add_ship_to company_name: 'Google Inc.', phone_number: '0207 031 3000', address_line_1: '1 St Giles High Street', city: 'London', state: 'England', postal_code: 'WC2H 8AG', country: 'GB'
  rate_builder.add_package weight: '0.5', unit: 'KGS'
end

response.status_description
# => "Success" 
response.error_description
# => NoMethodError (undefined method `[]' for nil:NilClass)

[aiperon]

@benschool , @matt423 Is there any chances of reviewing and merging?