Response code and body references is Error classes

• Hang onto the error code and body in exceptions

Interface change to StatisticsOpenApi#bandwidth_usage

• StatisticsOpenApi#bandwidth_usage returns 0.0 when the CDNetworks OpenAPI returns "404" (indicating no traffic data for a given PAD)

OpenAPI v2 statistics

• Integration with OpenAPI v2 statistics "edge traffic" endpoint for determining bandwidth usage.