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

Runtime error leaf class raised when a resource requested was not found. HTTP code: 404 (aka Not Found).

To handle specifically you would do the following:

 begin
   timeline = twitter.timeline_for(:friends, :since => tweet)
 rescue NotFoundError => nfe
   # do something to notify user that resource was not found.
 end

[Validate]