Class Twitter::ForbiddenError
In: lib/twitter/core.rb
Parent: RESTError

Runtime error leaf class raised when update limit reached. HTTP code: 403 (aka Forbidden).

To handle specifically you would do the following:

 begin
   timeline = twitter.timeline_for(:friends, :since => tweet)
 rescue ForbiddenError => fe
   # do something to notify user that update limit has been reached
 end

[Validate]