py3: cast error message to localstr in blackbox.py

According to the exceptions reporting mechanism, this is the  #1 crasher
in the test harness for Python 3.

Since this exception is in an except block, we'll likely get a
new  #1 crasher after this change. But at least we won't die
during error handling!

Differential Revision: https://phab.mercurial-scm.org/D1478

hgext/blackbox.py

@@ -44,6 +44,7 b' from mercurial.i18n import _'
 from mercurial.node import hex
 
 from mercurial import (
+    encoding,
     registrar,
     ui as uimod,
     util,
@@ -182,7 +183,7 b' def wrapui(ui):'
                     fp.write(fmt % args)
             except (IOError, OSError) as err:
                 self.debug('warning: cannot write to blackbox.log: %s\n' %
-                           err.strerror)
+                           encoding.strtolocal(err.strerror))
                 # do not restore _bbinlog intentionally to avoid failed
                 # logging again
             else: