Issue #268 (https://github.com/timmolter/XChange/issues/268):
-if the JSON returned from the server contains an "error" entry, deserialize this value and return it to the user.
-since it appears the server will sometimes return a single string containing an error message and other times may return an object containing an array of messages, a custom serializer was required to boil that down into a single string
Merge pull request #269 from senecaso/develop

Issue #268 (https://github.com/timmolter/XChange/issues/268):