Mercurial: errors, properly expose lookup exceptions traceback.

vcsserver/exceptions.py

@@ -37,7 +37,7 b' def _make_exception(kind, org_exc, *args'
     exc = Exception(*args)
     exc._vcs_kind = kind
     exc._org_exc = org_exc
-    exc._org_exc_tb = ''
+    exc._org_exc_tb = getattr(org_exc, '_org_exc_tb', '')
     return exc