Also try to resume on missed heartbeats

src/Discord.Net.WebSocket/DiscordSocketClient.cs

@@ -511,7 +511,7 @@ namespace Discord.WebSocket
                     case GatewayOpCode.Reconnect:
                         {
                             await _gatewayLogger.DebugAsync("Received Reconnect").ConfigureAwait(false);
-                            _connection.Error(new GatewayReconnectException());
+                            _connection.Error(new GatewayReconnectException("Server requested a reconnect"));
                         }
                         break;
                     case GatewayOpCode.Dispatch:
@@ -1689,7 +1689,7 @@ namespace Discord.WebSocket
                     {
                         if (ConnectionState == ConnectionState.Connected && (_guildDownloadTask?.IsCompleted ?? true))
                         {
-                            _connection.Error(new Exception("Server missed last heartbeat"));
+                            _connection.Error(new GatewayReconnectException("Server missed last heartbeat"));
                             return;
                         }
                     }

src/Discord.Net.WebSocket/GatewayReconnectException.cs

@@ -3,7 +3,7 @@ using System;
 namespace Discord.WebSocket
 {
     /// <summary>
-    /// An exception thrown when Discord requests the gateway client to
+    /// An exception thrown when the gateway client has been requested to
     /// reconnect.
     /// </summary>
     public class GatewayReconnectException : Exception
@@ -12,8 +12,11 @@ namespace Discord.WebSocket
         /// Creates a new instance of the
         /// <see cref="GatewayReconnectException"/> type.
         /// </summary>
-        public GatewayReconnectException()
-            : base("Server requested a reconnect")
+        /// <param name="message">
+        /// The reason why the gateway has been requested to reconnect.
+        /// </param>
+        public GatewayReconnectException(string message)
+            : base(message)
         { }
     }
 }