remind about missing values in age parsing error

bagel/utilities/pheno_utils.py

@@ -154,7 +154,8 @@ def transform_age(value: str, heuristic: str) -> float:
     except (ValueError, isodate.isoerror.ISO8601Error) as e:
         raise ValueError(
             f"There was a problem with applying the age transformation: {heuristic}. "
-            "Check that the specified transformation is correct for the age values in your data dictionary."
+            "Check that the transformation specified in the data dictionary is correct for the age values in your phenotypic file, "
+            "and that you correctly annotated any missing values in your age column."
         ) from e
     if not is_recognized_heuristic:
         raise ValueError(