ui: swallow EBADF on stderr stable

ui: swallow EBADF on stderr

ui.write_err already swallows EPIPE and EIO if a write to stderr fails.
On Mac OS X at least, a write to a closed file descriptor results in
EBADF. Before this patch, hg would exit with status 1 if a write to
stderr failed during startup (e.g. while trying to print a warning about
not finding an extension):

  $ ./hg --config extensions.foo= version 2>&-; echo $?
  1

With this patch, it correctly swallows stderr and continues to run the
command:

  $ ./hg --config extensions.foo= version 2>&-
  Mercurial Distributed SCM (version 2.1)
  ...

--- a/mercurial/ui.py	Thu Apr 05 12:31:21 2012 +0100
+++ b/mercurial/ui.py	Wed Apr 04 12:46:54 2012 -0500
@@ -480,7 +480,7 @@
             if not getattr(self.ferr, 'closed', False):
                 self.ferr.flush()
         except IOError, inst:
-            if inst.errno not in (errno.EPIPE, errno.EIO):
+            if inst.errno not in (errno.EPIPE, errno.EIO, errno.EBADF):
                 raise
 
     def flush(self):