Fix use of Porter for sampling

[derekpierre]

Type of PR:

• Bugfix
• Feature
• Documentation
• Other

Required reviews:

• 1
• 2
• 3

What this does:

High-level idea of the changes introduced in this PR.
List relevant API changes (if any), as well as related PRs and issues.

• Raise an exception whenever the request to Porter fails
• On testnets /get_ursulas is used instead of /bucket_sampling so we should adjust parsing of response appropriately.

Issues fixed/closed:

• Fixes #...

Why it's needed:

Explain how this PR fits in the greater context of the NuCypher Network.
E.g., if this PR address a nucypher/productdev issue, let reviewers know!

Notes for reviewers:

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[manumonti]

LGTM! 👌