Error docstrings make no sense.

[eugene-bulkin]

The docstrings in graphene.errors.query_errors don't make any sense.

See:

• https://github.com/PHB-CS123/graphene/blob/master/graphene/errors/query_errors.py#L7
• https://github.com/PHB-CS123/graphene/blob/master/graphene/errors/query_errors.py#L15
• https://github.com/PHB-CS123/graphene/blob/master/graphene/errors/query_errors.py#L19

These are either unclear or bad copy-pastes.