handle no privilege error

Popup an error window when don't have enough privilege to change
Registry.

shadowsocks-csharp/Controller/SystemProxy.cs

@@ -1,4 +1,5 @@
-using Microsoft.Win32;
+using System.Windows.Forms;
+using Microsoft.Win32;
 using System;
 using System.Collections.Generic;
 using System.Runtime.InteropServices;
@@ -13,32 +14,52 @@ namespace Shadowsocks.Controller
         public static extern bool InternetSetOption(IntPtr hInternet, int dwOption, IntPtr lpBuffer, int dwBufferLength);
         public const int INTERNET_OPTION_SETTINGS_CHANGED = 39;
         public const int INTERNET_OPTION_REFRESH = 37;
-        static bool settingsReturn, refreshReturn;
+        static bool _settingsReturn, _refreshReturn;
 
         public static void NotifyIE()
         {
             // These lines implement the Interface in the beginning of program 
             // They cause the OS to refresh the settings, causing IP to realy update
-            settingsReturn = InternetSetOption(IntPtr.Zero, INTERNET_OPTION_SETTINGS_CHANGED, IntPtr.Zero, 0);
-            refreshReturn = InternetSetOption(IntPtr.Zero, INTERNET_OPTION_REFRESH, IntPtr.Zero, 0);
+            _settingsReturn = InternetSetOption(IntPtr.Zero, INTERNET_OPTION_SETTINGS_CHANGED, IntPtr.Zero, 0);
+            _refreshReturn = InternetSetOption(IntPtr.Zero, INTERNET_OPTION_REFRESH, IntPtr.Zero, 0);
         }
 
         public static void Enable()
         {
-            RegistryKey registry = Registry.CurrentUser.OpenSubKey("Software\\Microsoft\\Windows\\CurrentVersion\\Internet Settings", true);
-            registry.SetValue("ProxyEnable", 0);
-            registry.SetValue("ProxyServer", "");
-            registry.SetValue("AutoConfigURL", "http://127.0.0.1:8090/pac?t=" + GetTimestamp(DateTime.Now));
-            SystemProxy.NotifyIE();
+            try
+            {
+                RegistryKey registry =
+                    Registry.CurrentUser.OpenSubKey("Software\\Microsoft\\Windows\\CurrentVersion\\Internet Settings",
+                        true);
+                registry.SetValue("ProxyEnable", 0);
+                registry.SetValue("ProxyServer", "");
+                registry.SetValue("AutoConfigURL", "http://127.0.0.1:8090/pac?t=" + GetTimestamp(DateTime.Now));
+                SystemProxy.NotifyIE();
+            }
+            catch (Exception)
+            {
+                MessageBox.Show("can not change registry!");
+                throw;
+            }
         }
 
         public static void Disable()
         {
-            RegistryKey registry = Registry.CurrentUser.OpenSubKey("Software\\Microsoft\\Windows\\CurrentVersion\\Internet Settings", true);
-            registry.SetValue("ProxyEnable", 0);
-            registry.SetValue("ProxyServer", "");
-            registry.SetValue("AutoConfigURL", "");
-            SystemProxy.NotifyIE();
+            try
+            {
+                RegistryKey registry =
+                    Registry.CurrentUser.OpenSubKey("Software\\Microsoft\\Windows\\CurrentVersion\\Internet Settings",
+                        true);
+                registry.SetValue("ProxyEnable", 0);
+                registry.SetValue("ProxyServer", "");
+                registry.SetValue("AutoConfigURL", "");
+                SystemProxy.NotifyIE();
+            }
+            catch (Exception)
+            {
+                MessageBox.Show("can not change registry!");
+                throw;
+            }
         }
 
         private static String GetTimestamp(DateTime value)