editor: simplify some error handling

openvpn_editor_new(), advanced_dialog_new_hash_from_connection(), and
advanced_dialog_new_hash_from_dialog() are not supposed to fail under
any normal operation.

Drop some error handling, plain g_return_*() are good enough.
2 jobs for th/auth-validation in 10 minutes (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #114791
fedora28_dist

00:03:27

 
  Test
passed #114792
fedora_from_dist

00:06:33