Printing out error's underlying reason (#1227)

Summary:
Pull Request resolved: #1227

Even though the root exception is added in the "raise from" clause, the logs don't show the underlying root cause.

It's faster to find problems if we print it out this way

Reviewed By: hikushalhere

Differential Revision: D19171878

fbshipit-source-id: 8cdb6337d7abedb5904b03e5841070828ae2a4fd

pytext/config/serialize.py

@@ -86,7 +86,11 @@ def _union_from_json(subclasses, json_obj):
         return _value_from_json(subclasses_dict[type_name], json_obj)
     except Exception as e:
         raise UnionTypeError(
-            f"failed to parse union {subclasses} from json payload {json_obj}"
+            (
+                f"failed to parse union {subclasses} from"
+                f"json payload {json_obj} \n"
+                f"Reason: {e}"
+            )
         ) from e