LU-3306 mdt: Do not LBUG on double set of err_serious

It seems the twisted code pathes in mdt could take strange routes
and it's quite hard to guarantee they won't pass through setting
err_serious twice, esp. since error handlers are traditionally
poorly tested.

Change-Id: I88fe880c22670263273bd79073efaecbe53d9215
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Reviewed-on: http://review.whamcloud.com/6383
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>