if logged in user is not active for 20 mins I want a message box to tell them they are logged out ??

 hi,
i am using membership on my site
If a user is not active then after 2o minutes they click a button an exception is raised and it goes to a red error page
which does not look good.
Instead I want to re-direct them to login page with a message saying you have been logged out.
 
thanks for any solutions
 
P
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