Distinguish TimeoutErrors for open and read timeouts #718

In faraday/adapter/rack.rb, TimeoutError is raised for both open and read timeouts:

timeout  = env[:request][:timeout] || env[:request][:open_timeout]
response = if timeout
  Timer.timeout(timeout, Faraday::Error::TimeoutError) { execute_request(env, rack_env) }
else ... end

According to https://stackoverflow.com/questions/10322283/what-is-timeout-and-open-timeout-in-faraday, open_timeout is for the tcp connection and timeout is for the response read.

It would be nice to have separate exception types for these timeouts. Then we could determine whether or not to retry the request. Does adding something like Faraday::Error::OpenTimeoutError and Faraday::Error::ResponseTimeoutError and using those here make sense?