DCOS: core.dcos_url unset is unhandled

[edwardcallahan]

If I run conduct dcos-setup but have not set dcos config set core.dcos_url on amd64, commands such as conduct info fail with

ubuntu@ip-10-100-1-96:~$ dcos conduct info
Error: HTTP Exception
Traceback (most recent call last):
  File "dcos/http.py", line 96, in _request
  File "requests/api.py", line 56, in request
  File "requests/sessions.py", line 488, in request
  File "requests/sessions.py", line 603, in send
  File "requests/sessions.py", line 685, in get_adapter
requests.exceptions.InvalidSchema: No connection adapters were found for 'b''://127.0.0.1:443/service/conductr/v2/bundles'
Error: Encountered unexpected error.
Error: Reason: DCOSException HTTP Exception: No connection adapters were found for 'b''://127.0.0.1:443/service/conductr/v2/bundles'
Error: Further information of the error can be found in the error log file: /home/ubuntu/.conductr/errors.log

Expected error message