Friendlier web UI message for large commits (BB-15156)

One common problem we have is people unknowingly pushing up very large commits / diffs to the server. After a certain size (or number of lines), we'll give up and throw a 500 error. This is related to: BCLOUD-7723 however, we're not looking for a solution – maybe just a better message to show people that their diff will not show on the UI due to the commit size