Expose HttpQueryError in __all__

`HttpQueryError` not part of `__all__` is a breaking change to libraries like `sanic-graphql` who rely on importing it:
https://github.com/graphql-python/sanic-graphql/blob/master/sanic_graphql/graphqlview.py#L12

Author: ekampf

graphql_server/__init__.py

@@ -203,6 +203,7 @@ def load_json_body(data):
 
 
 __all__ = [
+    "HttpQueryError",
     "default_format_error",
     "SkipException",
     "run_http_query",