[Cashtab] Guard Rewards and Agora fetches against non-OK responses

Summary: Check response.ok (and JSON parse failures on claim) before treating Rewards eligibility/claim and Agora blacklist bodies as success, matching the CoinGecko price-fetch pattern.

Test Plan:

• Manually: break rewards/blacklist URLs or return 500 HTML; Rewards shows the existing error toast; Agora falls back to the local blacklist
• Spot-check Rewards claim still surfaces structured JSON error fields from 4xx bodies

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D20348