Improve `sdk-lite` error handling

[Lakes41]

sdk-lite currently throws generic HTTP <status> errors when API requests fail. Add typed errors so frontend and integration clients can handle API failures more reliably.

Suggested tasks

• Add a GuildPassApiError class
• Include status code, request path, and safe response message
• Handle non-JSON error responses
• Handle empty successful responses safely
• Add tests for successful responses, API errors, and invalid JSON
• Update usage documentation if needed

Acceptance criteria

• API failures throw typed errors
• Consumers can inspect status code and message
• Non-JSON and empty responses do not crash unexpectedly
• Error handling is covered by tests

[Adesam007-pr3dator]

Hi, I would love to work on this issue