improve error handling for spotify api failures and private follows

[gorkie-agent]

while checking out the app/readme, i noticed the roadmap already calls out better error handling and private-follow handling.

suggestion: improve the app's failure states around spotify token expiry, private followers/following, and undocumented api errors so users get a clear message instead of a vague crash or partial graph.

possible acceptance criteria:

• show a friendly error when the token is expired/invalid
• distinguish private data from network/api failures
• keep the graph usable even when some lookups fail
• surface which node/request failed and why