If store used by throttle is down or returns an exception, optionally don't rate limit

Added a parameter, :proceed_on_error, that when set to true will not rate limit
requests if the store used by throttle is down or returns an exception. This
is to allow modes where request processing takes precedence over rate-limiting
when the back-end store goes down (currently if the store goes down, Exceptions
propogate up the stack and all requests are rate limited