stdio: raise StdioError if something goes wrong in ui._write_err

stdio: raise StdioError if something goes wrong in ui._write_err

The prior code used to ignore certain classes of error, which was
not the right thing to do.

--- a/mercurial/ui.py	Tue Apr 11 14:54:12 2017 -0700
+++ b/mercurial/ui.py	Tue Apr 11 14:54:12 2017 -0700
@@ -801,8 +801,7 @@
                 if not getattr(self.ferr, 'closed', False):
                     self.ferr.flush()
         except IOError as inst:
-            if inst.errno not in (errno.EPIPE, errno.EIO, errno.EBADF):
-                raise
+            raise error.StdioError(inst)
 
     def flush(self):
         # opencode timeblockedsection because this is a critical path