Don't crash on failure to obtain version info and inform user on what is needed to be able to retrieve version info from the Go client.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

WireGuardNetworkExtension/PacketTunnelProvider.swift

@@ -112,7 +112,9 @@ class PacketTunnelProvider: NEPacketTunnelProvider {
     override func handleAppMessage(_ messageData: Data, completionHandler: ((Data?) -> Void)?) {
         let responseData: Data?
 
-        switch ExtensionMessage(messageData) {
+        let message = ExtensionMessage(messageData)
+
+        switch message {
         case ExtensionMessage.requestVersion:
             responseData = (wgVersion().flatMap { String(cString: $0) } ?? "").data(using: .utf8)
         default: