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.