Give better error when include_webui=1 and webui can't be started. (#4471)

python/ray/services.py

@@ -924,10 +924,9 @@ def start_dashboard(redis_address,
         import aiohttp  # noqa: F401
         import psutil  # noqa: F401
     except ImportError:
-        logger.warning(
+        raise ImportError(
             "Failed to start the dashboard. The dashboard requires Python 3 "
             "as well as 'pip install aiohttp psutil'.")
-        return None, None
 
     process_info = start_ray_process(
         command,