feature: Support FailIfNotExists on MessageReference (#2283)

Fixes #2282

src/Discord.Net.Rest/API/Common/MessageReference.cs

@@ -12,5 +12,8 @@ namespace Discord.API
 
         [JsonProperty("guild_id")]
         public Optional<ulong> GuildId { get; set; }
+
+        [JsonProperty("fail_if_not_exists")]
+        public Optional<bool> FailIfNotExists { get; set; }
     }
 }

src/Discord.Net.Rest/Entities/Messages/RestMessage.cs

@@ -144,7 +144,8 @@ namespace Discord.Rest
                 {
                     GuildId = model.Reference.Value.GuildId,
                     InternalChannelId = model.Reference.Value.ChannelId,
-                    MessageId = model.Reference.Value.MessageId
+                    MessageId = model.Reference.Value.MessageId,
+                    FailIfNotExists = model.Reference.Value.FailIfNotExists
                 };
             }
 

src/Discord.Net.Rest/Extensions/EntityExtensions.cs

@@ -87,6 +87,7 @@ namespace Discord.Rest
                 ChannelId = entity.InternalChannelId,
                 GuildId = entity.GuildId,
                 MessageId = entity.MessageId,
+                FailIfNotExists = entity.FailIfNotExists
             };
         }
         public static IEnumerable<string> EnumerateMentionTypes(this AllowedMentionTypes mentionTypes)