fix: show exception class name when pool source error message is empty

TimeoutError.__str__() returns '' in Python, causing truncated log
lines like "source ... failed: ". Fall back to the class name.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/s5p/pool.py

@@ -171,7 +171,8 @@ class ProxyPool:
             src = self._cfg.sources[i]
             label = src.url or src.file or "?"
             if isinstance(result, Exception):
-                logger.warning("pool: source %s failed: %s", label, result)
+                err = str(result) or type(result).__name__
+                logger.warning("pool: source %s failed: %s", label, err)
             else:
                 kind = "fetched" if src.url else "loaded"
                 logger.info("pool: %s %d proxies from %s", kind, len(result), label)