[Benchmarks] Reduce ready checker log verbosity (#34349)

Signed-off-by: Tomas Ruiz <tomas.ruiz.te@gmail.com>

vllm/benchmarks/lib/ready_checker.py

@@ -66,7 +66,8 @@ async def wait_for_endpoint(
                     pbar.close()
                     return output
                 else:
-                    logger.warning("Endpoint is not ready. Error='%s'", output.error)
+                    err_last_line = str(output.error).rstrip().rsplit("\n", 1)[-1]
+                    logger.warning("Endpoint is not ready. Error='%s'", err_last_line)
             except aiohttp.ClientConnectorError:
                 pass