Improve exit IP failure diagnostics

app.py

@@ -436,7 +436,7 @@ async def get_exit_ip_via_socks(proxy: dict[str, Any], timeout: float) -> str |
             errors.append(f"{host}: no ip in response")
         except Exception as e:
             errors.append(f"{host}: {e}")
-    proxy["last_exit_ip_error"] = "; ".join(errors[-3:])
+    proxy["last_exit_ip_error"] = "; ".join(errors[-3:]) or "exit ip lookup failed: empty response from all providers"
     return None
 
 
@@ -447,7 +447,7 @@ async def check_proxy(proxy: dict[str, Any]) -> tuple[bool, int | None, str | No
         await socks5_connect(proxy, timeout)
         latency = int((time.perf_counter() - start) * 1000)
         exit_ip = await get_exit_ip_via_socks(proxy, timeout)
-        err = None if exit_ip else proxy.get("last_exit_ip_error")
+        err = None if exit_ip else (proxy.get("last_exit_ip_error") or "exit ip lookup failed")
         return True, latency, err, exit_ip
     except Exception as e:
         return False, int((time.perf_counter() - start) * 1000), str(e), None