Better error message for create Link object

Every time I create a link with `targetId` that point to an entity, and set `parent` to the parent of `targetId`, I get the similar error:

I think this is related to the naming collision within the same parent. However, the error was not helpful.

On the web client, I would see the error:

Status Code: 409 message: An entity with the name: test.txt already exists with a parentId: syn11801732

But in the Python client, we see the error above. It looks like the Python client tried to update the target entity to a link instead of creating a link to the target entity.

https://github.com/Sage-Bionetworks/synapsePythonClient/pull/590

, would you be able to validate this issue by Friday?

, I am asking to validate this issue so that we can release the python client version 1.9.0 today. If you find that this is not fixed to your satisfaction, please reopen later.