add service unavailable handling

Author: josler

lib/intercom/request.rb

@@ -110,6 +110,8 @@ def raise_application_errors_on_failure(error_list_details, http_code)
         raise Intercom::ResourceNotFound.new(error_details['message'], error_context)
       when "rate_limit_exceeded"
         raise Intercom::RateLimitExceeded.new(error_details['message'], error_context)
+      when 'service_unavailable'
+        raise Intercom::ServiceUnavailableError.new(error_details['message'], error_context)
       when nil, ''
         raise Intercom::UnexpectedError.new(message_for_unexpected_error_without_type(error_details, parsed_http_code), error_context)
       else