identity: don't register new identity to list if sign in unsuccessful

If we add an identity to the list when sign in fails, then we're going
to prune it from the list immediately resulting in an identity-removed
signal right away. This will leads to an ensure credentials call
(which is needed in most cases on identity removal to synchronize state
 with the daemon, but not in this case), which may cause it to sign
in again. This can lead to a sign in attempt every 5 seconds.

https://bugzilla.gnome.org/show_bug.cgi?id=768808