Code review comment for lp:~evfool/software-properties/lp1061101

I have updated the branch to raise a SigningKeyException, and handled it in apt-add-repository and software-properties-gtk command line option, however when using the interface and calling add_source_from_line method over dbus the exception gets swallowed somehow.
If anyone with a bit more DBUS experience could take a look, it would be nice. Right now there's no feedback when the signing key cannot be added from the GTK interface, but this did happened before the fix too, so there's no change there.