c-e: improve error message for non-existing certificate file

When the connection references a certifiate file that does not exist,
the GUI's file picker button shows an "(None)", however the "Save"
button is disable with message:

  "Invalid setting Wi-Fi Security: invalid EAP-PEAP CA certificate: unspecified error validating eap-method file"

Slightly improve that by showing instead

  "Invalid setting Wi-Fi Security: invalid EAP-PEAP CA certificate: file "..." does not exist"

The solution is not optimal because the GUI shows file "(None)",
with is some hidden information that makes the connection invalid.

https://bugzilla.gnome.org/show_bug.cgi?id=780423
(cherry picked from commit b603844f)