Moved all error strings to src/errors/messages and a few other things (#1695)

* Added missing error messages
As well as `Guild#setRolePosition` and `Guild#setChannelPosition`'s first arg validation
And fixed a typo in `Guild#setChannelPosition`
`roles` -> `channels`

* Reverted collection and Util constructors

* Removed leftover messages
Should have been in the second commit.

* It's a single invalid permission and removed unused flag error

* Fix INVALID_TOKEN -> TOKEN_INVALID as of #1703

src/structures/User.js

@@ -3,6 +3,7 @@ const Constants = require('../util/Constants');
 const { Presence } = require('./Presence');
 const UserProfile = require('./UserProfile');
 const Snowflake = require('../util/Snowflake');
+const { Error } = require('../errors');
 
 /**
  * Represents a user on Discord.
@@ -212,7 +213,7 @@ class User {
    * @returns {Promise<DMChannel>}
    */
   deleteDM() {
-    if (!this.dmChannel) return Promise.reject(new Error('No DM Channel exists!'));
+    if (!this.dmChannel) return Promise.reject(new Error('USER_NO_DMCHANNEL'));
     return this.client.api.channels[this.dmChannel.id].delete()
       .then(data => this.client.actions.ChannelDelete.handle(data).channel);
   }