util: raise with traceback in Python 2

Summary: Similar to D23819023 (c96de76ac0) but works on Python 2, too.

Reviewed By: DurhamG

Differential Revision: D23858273

fbshipit-source-id: b15be07c8657bc8cb37960b631f2b31e4a78892b

eden/scm/edenscm/mercurial/util.py

@@ -4357,6 +4357,9 @@ def threaded(func):
 
         variant, value = result
         if variant == "err":
+            tb = getattr(value, "__traceback__", None)
+            if tb is not None:
+                pycompat.raisewithtb(value, tb)
             raise value
 
         return value