This is not that rare on TRV applications. What we usually suggest is to just file a new application adding this information to the explanation letter (and any other relevant support documentation). This is way smoother and faster than going through an appeal process and has much more rate of success that trying to show CIC made a mistake.
The only benefit of having CIC "reconsider" is that you won't have to answer yes when asked "have you ever been refused a visa?".... which can be easily explained by "yes, not enough support documentation, but granted after second submission" and get on with your life.