[Rackspace|Compute] updated ready method on Image to raise an exception if an error state occurs. This is similar to the behavior of Server.

lib/fog/rackspace/models/compute_v2/image.rb

@@ -46,9 +46,14 @@ module Fog
           metadata.from_hash(hash)
         end
 
-        def ready?
-          state ==  ACTIVE
+        def ready?(ready_state = ACTIVE, error_states=[ERROR])
+          if error_states
+            error_states = Array(error_states)
+            raise "Image should have transitioned to '#{ready_state}' not '#{state}'" if error_states.include?(state)
+          end
+          state == ready_state
         end
+        
 
         def destroy
           requires :identity