Improve error messages and exception mapping

[subzeroid]

Summary

Even when the library cannot fully fix an Instagram-side problem, it should still fail with errors that are understandable and actionable.

This issue tracks improving error messages and exception mapping in the highest-frequency failure paths.

Problem areas

Repeated reports show confusion around:

• low-context exceptions during login and challenge resolution
• ambiguous upload failures
• unclear fallback-related public/private errors
• errors that do not explain whether the problem is configuration, account state, proxy quality, or an unsupported Instagram flow

Proposed scope

• review the most common high-noise exceptions
• improve mapping from raw API responses to actionable exceptions
• preserve useful context without leaking low-level noise
• align error text with the likely operator action

Definition of done

• frequent failure paths produce clearer user-facing errors
• raw low-context exceptions are reduced
• docs and tests cover the improved messages where appropriate