core: fix CA cert mishandling after cert file deletion (deb #560067) (rh #546793)

If a connection was created with a CA certificate, but the user later
moved or deleted that CA certificate, the applet would simply provide the
connection to NetworkManager without any CA certificate.  This could cause
NM to connect to the original network (or a network spoofing the original
network) without verifying the identity of the network as the user
expects.

In the future we can/should do better here by (1) alerting the user that
some connection is now no longer complete by flagging it in the connection
editor or notifying the user somehow, and (2) by using a freaking' cert
store already (not that Linux has one yet).