Show clear message when the same study name is existed on advisor server

[stvreumi]

If the study name of the client is existed on the server, the client will output following error.

Traceback (most recent call last):
File "example.py", line 28, in <module>
  study = client.create_study(name, study_configuration)
File "/Users/stvreumi/dev/crypto/lib/python3.6/site-packages/advisor_client/client.py", line 36, in create_study
  study = Study.from_dict(response.json()["data"])
KeyError: 'data'

Because of the response only have following key and value
{'error': 'The study Study exists'}

Maybe client can show clear message to user that using same study name causes error.