Add error handler for Eris errors

Previously this was done within Knub, but Knub no longer
automatically places an error handler for Eris since v30.0.0-beta.24

backend/src/index.ts

@@ -147,6 +147,10 @@ connect().then(async () => {
     }
   });
 
+  client.on("error", err => {
+    logger.error(`[ERIS] ${String(err)}`);
+  });
+
   const allowedGuilds = new AllowedGuilds();
   const guildConfigs = new Configs();