Legacy VPN Configuration Dialog includes error message if VPN is an

insecure type.

Screenshot: https://screenshot.googleplex.com/53pAJuhTDLJW4Em
Bug: 176821216
Test: atest SettingsUnitTests

Change-Id: I528d46654a39df04b647d6d0137aaf80e7adb05c
Merged-In: I528d46654a39df04b647d6d0137aaf80e7adb05c
(cherry picked from commit 5d92ad52)