Manage url and http exceptions for non-existent entities

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Entity does not exist so the JSON is not returned by wikidata.

When an http or url exception is raised by a bad request, the entity data is set to none.

An empty entity is returned with an id that does not exist.

Fixes #8 .

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Something is wrong with $GITHUB_ACCESS_TOKEN?

CI fails for python 3.6 .

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• I added a test to tests/client_test.py
• Rebased with master
• Python build fails in CI

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@dahlia Implemented your suggested changes for non-existent entities, all checks pass

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Thanks for your contribution!

Thanks for the guidance on better code! Much appreciated.