intelligible errors on schema creation.

When creating a schema, if there is a problem with one of your QueryObjects, the resulting error message and traceback does not give any indication as to which ObjectType class is broken.

ex)
```
Traceback (most recent call last):
File "", line 1, in
File "/home/user/sonicsd/src/api/graphql.py", line 30, in schema
File "/usr/local/lib/python3.7/dist-packages/graphene/types/schema.py", line 78, in __init__
self.build_typemap()
File "/usr/local/lib/python3.7/dist-packages/graphene/types/schema.py", line 168, in build_typemap
initial_types, auto_camelcase=self.auto_camelcase, schema=self
File "/usr/local/lib/python3.7/dist-packages/graphene/types/typemap.py", line 80, in __init__
super(TypeMap, self).__init__(types)
File "/usr/local/lib/python3.7/dist-packages/graphql/type/typemap.py", line 31, in __init__
self.update(reduce(self.reducer, types, OrderedDict())) # type: ignore
File "/usr/local/lib/python3.7/dist-packages/graphene/types/typemap.py", line 88, in reducer
return self.graphene_reducer(map, type)
File "/usr/local/lib/python3.7/dist-packages/graphene/types/typemap.py", line 117, in graphene_reducer
return GraphQLTypeMap.reducer(map, internal_type)
File "/usr/local/lib/python3.7/dist-packages/graphql/type/typemap.py", line 109, in reducer
field_map = type_.fields
File "/usr/local/lib/python3.7/dist-packages/graphql/pyutils/cached_property.py", line 22, in __get__
value = obj.__dict__[self.func.__name__] = self.func(obj)
File "/usr/local/lib/python3.7/dist-packages/graphql/type/definition.py", line 198, in fields
return define_field_map(self, self._fields)
File "/usr/local/lib/python3.7/dist-packages/graphql/type/definition.py", line 217, in define_field_map
).format(type_)
AssertionError: Query fields must be a mapping (dict / OrderedDict) with field names as keys or a function which returns such a mapping.
```

In the error there should at least be the name of the Query class whose fields are invalid.
In this case I believe the error turned out to be that the root Query had no fields because I was improperly dynamically generating them.