Fixes #1251 jsonp timeout raises two errors

More generally, I have the impression that, in the error case, not everything is properly cleaned.
Especially, `delete this[request.evalCb];` which is done in `_onJsonPLoad` is not called in case of an error and the the callback probably stays on the `aria.core.transport.JsonP`.
I think there should be one function which does all the clean-up, and that all callbacks should call it.