Fixed DataStore, deprecation leftovers and a bit of Event Constants (#1841)

* Fixed leftover fetchThing and removed unused methods/error messages

* Added resume event constant and used event constants wherever possible

* Replaced mentions of removed method name with their new name.

* Fixed typo: resume -> resumed

src/structures/TextChannel.js

@@ -73,9 +73,6 @@ class TextChannel extends GuildChannel {
   // These are here only for documentation purposes - they are implemented by TextBasedChannel
   /* eslint-disable no-empty-function */
   send() {}
-  fetchMessage() {}
-  fetchMessages() {}
-  fetchPinnedMessages() {}
   search() {}
   startTyping() {}
   stopTyping() {}