fix exception raising (#6901)

Author: kandluis

pytorch_lightning/loggers/tensorboard.py

@@ -207,7 +207,7 @@ def log_metrics(self, metrics: Dict[str, float], step: Optional[int] = None) ->
                 # todo: specify the possible exception
                 except Exception as ex:
                     m = f'\n you tried to log {v} which is not currently supported. Try a dict or a scalar/tensor.'
-                    type(ex)(ex.message + m)
+                    raise ValueError(m) from ex
 
     @rank_zero_only
     def log_graph(self, model: LightningModule, input_array=None):