improve error messages

views/index.pug

@@ -13,6 +13,8 @@ html
         if http_status_code == "401" || http_status_code == "503"
           p This error is probably caused because Reddit itself is down or having server issues.
           p Checking <a href="https://www.redditstatus.com">https://www.redditstatus.com</a> might give some information.
+        if http_status_code == "404"
+          p The resource you were looking for was not found.
     else
       - var subreddit = ''
       if(user_preferences.subbed_subreddits && Array.isArray(user_preferences.subbed_subreddits))

views/subreddit.pug

@@ -80,7 +80,8 @@ html
                 li(class=past === 'all' ? 'active' : '')
                   a(href="?t=all") all time
           if json.links.length === 0
-            p nothing here
+            .reddit-error
+              p This subreddit either doesn't exist, or any posts weren't found.
           else
             each link in json.links
               .link

views/subreddits_explore.pug

@@ -34,7 +34,8 @@ html
           input(type="submit", value="search")
       #links.sr.explore
         if json.links.length === 0
-          p nothing here
+          .reddit-error
+            p This subreddit either doesn't exist, or any posts weren't found.
         else
           .infobar.explore
             p click the <code>subscribe</code> or unsubscribe buttons to choose which subreddits appear on the home feed.