Ask user to refresh is user session if he just installed ubridge

Ref https://github.com/GNS3/gns3-server/issues/796

gns3server/compute/base_node.py

@@ -527,7 +527,7 @@ class BaseNode:
         """
 
         if self.ubridge_path is None:
-            raise NodeError("uBridge is not available or path doesn't exist")
+            raise NodeError("uBridge is not available or path doesn't exist. Or you just install GNS3 and need to restart your user session to refresh user permissions.")
 
         if not self._manager.has_privileged_access(self.ubridge_path):
             raise NodeError("uBridge requires root access or capability to interact with network adapters")