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

python/ray/node.py

@@ -316,8 +316,8 @@ class Node(object):
             self.all_processes[ray_constants.PROCESS_TYPE_DASHBOARD] = [
                 process_info
             ]
-        redis_client = self.create_redis_client()
-        redis_client.hmset("webui", {"url": self._webui_url})
+            redis_client = self.create_redis_client()
+            redis_client.hmset("webui", {"url": self._webui_url})
 
     def start_plasma_store(self):
         """Start the plasma store."""

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,