Fix retry logic for HTTP 5xx errors and network failures

Refactors error handling to retry all 5xx errors (not just 502), network errors (URLError, socket.error, IncompleteRead), and JSON parse errors with exponential backoff and jitter. Respects retry-after and rate limit headers per GitHub API requirements. Consolidates retry logic into make_request_with_retry() wrapper and adds clear logging for retry attempts and failures. Removes dead code from 2016 (errors list, _request_http_error, _request_url_error) that was intentionally disabled in commit 1e5a904 to fix #29.

Fixes #140, #110, #138

Author: Iamrodos