Add SchemaDocumentError to __all__, instead of 2x SchemaDocument

From what I can tell, it was intended to include `SchemaDocumentError`
in `__all__`, but instead two instances of `SchemaDocument` were listed.
To help clear it up, I also sorted `__all__`.

Now, `SchemaDocumentError` can be directly imported. Since this class
is the parent of all schema document errors, you can use this class
in a try/catch to pick out any schema document error.