if the category doesn't exist, give an error instead of not replying

systems/quote.rb

@@ -29,13 +29,16 @@ module QuoteSystem
       quotes = db.execute("SELECT * FROM quotes")
       quote = quotes.sample
       event.channel.send_message("##{quote[0]} <#{quote[2]}>: #{quote[3]}")
-      nil
     else
       quotes = db.execute("SELECT * FROM quotes WHERE category = ?", [category])
+      if quotes == nil
+        event.channel.send_message("Uh, this is awkward. The category you're trying to get a quote from doesn't exist.")
+        return
+      end
       quote = quotes.sample
       event.channel.send_message("##{quote[0]}: #{quote[3]}")
-      nil
     end
+    nil
   end
 
   command :quotecategories do |event, *message|