Throw when attempting to add or remove a member's EveryoneRole (#781)

* Throw when attempting to add or remove a member's EveryoneRole

This resolves #780

* Removed braces

src/Discord.Net.Core/Utils/Preconditions.cs

@@ -192,5 +192,13 @@ namespace Discord
                     throw new ArgumentOutOfRangeException(name, "Messages must be younger than two weeks old.");
             }
         }
+        public static void NotEveryoneRole(ulong[] roles, ulong guildId, string name)
+        {
+            for (var i = 0; i < roles.Length; i++)
+            {
+                if (roles[i] == guildId)
+                    throw new ArgumentException($"The everyone role cannot be assigned to a user", name);
+            }
+        }
     }
 }