ui: extract buffer write from protect and timed 'write_err' output

That subcall to 'self.write' is never doing actual write but only store things
in buffers. So we do not need to protect it for exception not to time its
execution.

This will make it easier to extract a '_write_err' function as we did for
'write'.

mercurial/ui.py

@@ -822,9 +822,9 b' class ui(object):'
 
     def write_err(self, *args, **opts):
         self._progclear()
+        if self._bufferstates and self._bufferstates[-1][0]:
+            return self.write(*args, **opts)
         try:
-            if self._bufferstates and self._bufferstates[-1][0]:
-                return self.write(*args, **opts)
             with self.timeblockedsection('stdio'):
                 if not getattr(self.fout, 'closed', False):
                     self.fout.flush()