Output more information on failure of preparing VpnService

Fix #1424.

mobile/src/main/res/values/strings.xml

@@ -72,6 +72,7 @@
   <string name="stop">Stop</string>
   <string name="stopping">Shutting down…</string>
   <string name="vpn_error">%s</string>
+  <string name="vpn_permission_denied">Permission denied to create a VPN service</string>
   <string name="reboot_required">Failed to start VPN service. You might need to reboot your device.</string>
   <string name="profile_invalid_input">No valid profile data found.</string>
 

mobile/src/main/scala/com/github/shadowsocks/MainActivity.scala

@@ -167,7 +167,9 @@ class MainActivity extends Activity with ServiceBoundContext with Drawer.OnDrawe
 
   override def onActivityResult(requestCode: Int, resultCode: Int, data: Intent): Unit = resultCode match {
     case Activity.RESULT_OK => Utils.startSsService(this)
-    case _ => Log.e(TAG, "Failed to start VpnService")
+    case _ =>
+      Snackbar.make(findViewById(R.id.snackbar), R.string.vpn_permission_denied, Snackbar.LENGTH_LONG).show()
+      Log.e(TAG, "Failed to start VpnService: %s".formatLocal(Locale.ENGLISH, data.toString))
   }
 
   override def onCreate(savedInstanceState: Bundle) {