Check connection validity when creating the client

[reckart]

Is your feature request related to a problem? Please describe. When I create the client with the wrong URL, there can be cryptic error messages like:

__request(self, method, endpoint, **kwargs)
    520         raw_response = self._run_request(method, endpoint, **kwargs)
    521 
--> 522         response = raw_response.json()
    523         self.__handle_error(response)
    524         return response

These indicate that the server responds but does not provide JSON.

Describe the solution you'd like

• Already when creating the client, an initial request for the build info should be made to validate that the remote side exists and is alive
• When the server does not return JSON, a more sensible error message should be generated

[reckart]

Actually, one can just call client.ensureAvailable().