Do not write full backctrace to the log if a controlled error is raised during a scheduled feed update.

The full backctrace messes up the log and adds no value in this case.
Same as commit 983cc1d98b34515fda7d860b2bfede4943d726e3 when manually refreshing feeds.
Same as 983cc1d98b34515fda7d860b2bfede4943d726e3 but when subscribing a user to a feed.