be more specific for error message (#1409)

### What problem does this PR solve?

#918 

### Type of change

- [x] Refactoring

graph/component/base.py

@@ -19,7 +19,7 @@ import json
 import os
 from copy import deepcopy
 from functools import partial
-from typing import List, Dict
+from typing import List, Dict, Tuple, Union
 
 import pandas as pd
 
@@ -246,7 +246,7 @@ class ComponentParamBase(ABC):
     def check_empty(param, descr):
         if not param:
             raise ValueError(
-                descr + " {} not supported empty value."
+                descr + " does not support empty value."
             )
 
     @staticmethod
@@ -411,12 +411,23 @@ class ComponentBase(ABC):
     def _run(self, history, **kwargs):
         raise NotImplementedError()
 
-    def output(self) -> pd.DataFrame:
+    def output(self, allow_partial=True) -> Tuple[str, Union[pd.DataFrame, partial]]:
         o = getattr(self._param, self._param.output_var_name)
         if not isinstance(o, partial) and not isinstance(o, pd.DataFrame):
             if not isinstance(o, list): o = [o]
             o = pd.DataFrame(o)
-        return self._param.output_var_name, o
+
+        if allow_partial or not isinstance(o, partial):
+            if not isinstance(o, partial) and not isinstance(o, pd.DataFrame):
+                return pd.DataFrame(o if isinstance(o, list) else [o])
+            return self._param.output_var_name, o
+
+        outs = None
+        for oo in o():
+            if not isinstance(oo, pd.DataFrame):
+                outs = pd.DataFrame(oo if isinstance(oo, list) else [oo])
+            else: outs = oo
+        return self._param.output_var_name, outs
 
     def reset(self):
         setattr(self._param, self._param.output_var_name, None)
@@ -446,7 +457,7 @@ class ComponentBase(ABC):
             if self.component_name.lower().find("answer") >= 0:
                 if self.get_component_name(u) in ["relevant"]: continue
 
-            upstream_outs.append(self._canvas.get_component(u)["obj"].output()[1])
+            else: upstream_outs.append(self._canvas.get_component(u)["obj"].output(allow_partial=False)[1])
             break
 
         return pd.concat(upstream_outs, ignore_index=False)