Set ErrorMessage on errors

[zglazer]

Using the SDK to create or update an object, we get error codes in the result but no accompanying error messages. The error message is in the CreateResult object for a DataExtensionRowCreate operation but we never actually get that message back in ETResult when using the SDK. In fact, I don't see ErrorMessage ever being set anywhere. The error codes themselves are not descriptive enough to identify the problem. Currently we have to leave INFO level logging on to see the actual error message.

The errorMessage field on ETResult should contain any and all errors (KeyError or otherwise).