No feed log level

This patch drops the log level for logging requests to non-existing
feeds from error to debug. The reasoning behind this is that this is a
client error and a client error is returned and can be handled by the
client.

Otherwise anyone can easily spam the logs with error messages by just
requesting non-existing feeds.

server/managers/RssFeedManager.js

@@ -33,7 +33,7 @@ class RssFeedManager {
   async getFeed(req, res) {
     var feed = this.feeds[req.params.id]
     if (!feed) {
-      Logger.error(`[RssFeedManager] Feed not found ${req.params.id}`)
+      Logger.debug(`[RssFeedManager] Feed not found ${req.params.id}`)
       res.sendStatus(404)
       return
     }