Make validator raise for status

[krivard]

Description

Type of change (bug fix, new feature, etc), brief description, and motivation for these changes.

• makes validator raise for status when encountering server errors instead of trying to parse the server error as JSON
• adds requests-mock library, which seems worth it
• leaves TODOs to convert the remaining manual requests mocks

Changelog

Itemize code/test/documentation changes and files added/removed.

• validator datafetcher.py: raise_for_status on all requests
• status.py: include requests-mock library
• tests/validator/test_datafetcher.py: use requests-mock to properly mock a request that fails with an HTTP error code; ensure such responses are handled properly by the datafetcher code

Fixes

• Fixes bad error messages from validator when encountering server errors

[rzats]

LGTM