Add more granurality to client errors.

* Add specific exception for invalid number and international sms unavailable errors.
* Raise these exceptions in the case the error code is 21401 or 21408 respectively.

lib/twilio-ruby/rest/client.rb

@@ -258,7 +258,11 @@ def connect_and_send(request) # :doc:
           end
         end
         if response.kind_of? Net::HTTPClientError
-          raise Twilio::REST::RequestError, object['message']
+          case object['code']
+            when 21401 then raise Twilio::REST::InvalidNumber, object['message']
+            when 21408 then raise Twilio::REST::InternationalSmsUnavailable, object['message']
+            else raise Twilio::REST::RequestError, object['message']
+            end
         end
         object
       end

lib/twilio-ruby/rest/errors.rb

@@ -1,6 +1,8 @@
 module Twilio
   module REST
-    class RequestError < StandardError; end
     class ServerError < StandardError; end
+    class RequestError < StandardError; end
+    class InvalidNumber < RequestError; end
+    class InternationalSmsUnavailable < RequestError; end
   end
 end