fix: proper exception chaining

Author: gsong

epo_ops/utils.py

@@ -35,5 +35,5 @@ def validate_date(date):
     try:
         datetime.strptime(date, "%Y%m%d")
         return date
-    except ValueError:
-        raise InvalidDate("{0} is not a valid YYYYMMDD date.".format(date))
+    except ValueError as exc:
+        raise InvalidDate("{0} is not a valid YYYYMMDD date.".format(date)) from exc