py3: use stringutil.forcebytestr() instead of str()

We need to convert errors to bytes using stringutil.forcebytestr()

Differential Revision: https://phab.mercurial-scm.org/D3324

mercurial/exchange.py

@@ -2291,7 +2291,7 b' def filterclonebundleentries(repo, entri'
                     continue
 
             except error.InvalidBundleSpecification as e:
-                repo.ui.debug(str(e) + '\n')
+                repo.ui.debug(stringutil.forcebytestr(e) + '\n')
                 continue
             except error.UnsupportedBundleSpecification as e:
                 repo.ui.debug('filtering %s because unsupported bundle '