Don't crash if we are unable to set auto-start

This shouldn't happen, as it means that something allowed us to write
to the registry key on startup, but is denying us access when the settings
are saved. Show a MessageBox in this case.

Fixes #407

src/SyncTrayzor/Pages/Settings/SettingsViewModel.cs

@@ -392,8 +392,19 @@ public void Save()
 
             if (this.autostartProvider.CanWrite)
             {
-                var autostartConfig = new AutostartConfiguration() { AutoStart = this.StartOnLogon, StartMinimized = this.StartMinimized };
-                this.autostartProvider.SetAutoStart(autostartConfig);
+                // I've seen this fail, even though we successfully wrote on startup
+                try
+                {
+                    var autostartConfig = new AutostartConfiguration() { AutoStart = this.StartOnLogon, StartMinimized = this.StartMinimized };
+                    this.autostartProvider.SetAutoStart(autostartConfig);
+                }
+                catch
+                {
+                    this.windowManager.ShowMessageBox(
+                        Resources.SettingsView_CannotSetAutoStart_Message,
+                        Resources.SettingsView_CannotSetAutoStart_Title,
+                        MessageBoxButton.OK, MessageBoxImage.Error);
+                }
             }
 
             if (this.settings.Any(x => x.HasChanged && x.RequiresSyncTrayzorRestart))

src/SyncTrayzor/Properties/Resources.resx

@@ -985,4 +985,10 @@ Please donate to my charity fundraising campaign.</value>
     <value>Note:</value>
     <comment>Text shown in bold just before SettingsView_WatchedFolderEnabledInSyncthing</comment>
   </data>
+  <data name="SettingsView_CannotSetAutoStart_Message" xml:space="preserve">
+    <value>Unable to set your auto-start settings. This is probably due to anti-virus software.</value>
+  </data>
+  <data name="SettingsView_CannotSetAutoStart_Title" xml:space="preserve">
+    <value>Unable to set auto-start</value>
+  </data>
 </root>