Updated RateRequest WSDL from v8 to v16

As I needed SmartPost features present in v16, updated the WSDL and the examples for compatibility with elements that have been removed and new required elements that were added.

Also improved error handling in case of validation errors on suds' part. If suds attaches details to the exception, these will now also be printed out (usually this is a reference to the element failing validation). These kinds of errors were otherwise awfully hard to debug.

This is confirmed to work with retrieving SmartPost rates, I can add an example request in a separate commit if there's any demand for it.