android: Show an error if client certificate is unavailable

android: Show an error if client certificate is unavailable

This can happen on systems (e.g. Android 7.x) where Always-on VPNs are
triggered right after booting before the KeyChain is unlocked by the user.
Retrieving the certificate chain or private key then fails with
"KeyChainException: IllegalStateException: keystore is LOCKED" until the
user unlocks the screen once.

The built-in client actually also fails in this situation (e.g. with XAuth
RSA), it tries three times then stops and shows an error notification.