py3: convert exception to bytes to pass into ui.warn()

Here encoding.strtolocal() is used because exc maybe an IOError which could
contain a valid non-ascii unicode.

mercurial/hook.py

@@ -13,6 +13,7 b' import sys'
 from .i18n import _
 from . import (
     demandimport,
+    encoding,
     error,
     extensions,
     pycompat,
@@ -97,7 +98,7 b' def _pythonhook(ui, repo, htype, hname, '
                          (hname, exc.args[0]))
         else:
             ui.warn(_('error: %s hook raised an exception: '
-                           '%s\n') % (hname, exc))
+                      '%s\n') % (hname, encoding.strtolocal(str(exc))))
         if throw:
             raise
         if not ui.tracebackflag: