Arvados

Just a thought -- in this situation is it possible to raise an exception in the Python SDK that is both machine-readable (recognizable exception class) and human-readable ("you need to set your API_HOST and TOKEN vars or using a settings file")? It'd be nice to handle this such that humans get useful feedback even from applications that haven't wrapped the error with their own advice.

13692-acr-error @ 3a7481703542b0c95d5a23cb45b0d2e7c3ac4c9e

https://ci.curoverse.com/job/developer-run-tests/785/

Improves error messages emitted by the Python SDK, and adds an extra API call early in a-c-r initialization so that errors get reported early.

Example errors:

$ arvados-cwl-runner   --api=containers --submit-runner-ram=3333 --create-workflow submit_wf.cwl ../submit_test_job.json 
2018-07-05 17:03:40 arvados.cwl-runner ERROR: ARVADOS_API_TOKEN not set.
Please set in /home/peter/.config/arvados/settings.conf or export environment variable.

$ arvados-cwl-runner   --api=containers --submit-runner-ram=3333 --create-workflow submit_wf.cwl ../submit_test_job.json 
2018-07-05 17:01:48 arvados.cwl-runner ERROR: Could not connect to https://172.17.0.2:8000/arvados/v1/users/current?alt=json
[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:661)
Possible causes: remote SSL/TLS certificate expired, or was issued by an untrusted certificate authority.

This LGTM, please merge.