TMX Loader: Raise exceptions for unsupported formats

Currently, a NullPointerException is thrown if the 'map/layer/data' tag has no 'encoding' attribute.
No 'encoding' attribute means that XML encoding is used, and this is not supported by the loader.
I also added a different exception for when there is an 'encoding' value that the loader does not recognise (for future encodings).
I also added a minor comment.