[apibrk] change: Specify WebSocket close code (#1500)

* API breaking change: Specify WebSocket close code

Should fix #1479 and help overall with resuming sessions.

* Also try to resume on missed heartbeats

src/Discord.Net.WebSocket/GatewayReconnectException.cs

@@ -0,0 +1,22 @@
+using System;
+
+namespace Discord.WebSocket
+{
+    /// <summary>
+    /// An exception thrown when the gateway client has been requested to
+    /// reconnect.
+    /// </summary>
+    public class GatewayReconnectException : Exception
+    {
+        /// <summary>
+        /// Creates a new instance of the
+        /// <see cref="GatewayReconnectException"/> type.
+        /// </summary>
+        /// <param name="message">
+        /// The reason why the gateway has been requested to reconnect.
+        /// </param>
+        public GatewayReconnectException(string message)
+            : base(message)
+        { }
+    }
+}