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

Runtime error leaf class raised when client has reached rate limits. HTTP code: 400 (aka Bad Request).

To handle specifically you would do the following:

 begin
   timeline = twitter.timeline_for(:friends, :since => tweet)
 rescue RateLimitError => rlre
   # do something here...
 end

[Validate]