FIX-modin-project#2254: Fixed AttributeError not being thrown from ge…

…tattr

Signed-off-by: Gregory Shimansky <gregory.shimansky@intel.com>

modin/pandas/groupby.py

@@ -390,7 +390,9 @@ def _reconstruct_func(func, **kwargs):
                 **kwargs,
             )
         elif isinstance(func, str):
-            agg_func = getattr(self, func, None)
+            # Using "getattr" here masks possible AttributeError which we throw
+            # in __getattr__, so we should call __getattr__ directly instead.
+            agg_func = self.__getattr__(func)
             if callable(agg_func):
                 return agg_func(*args, **kwargs)