Wait for name acquisition before we register and emit notifications

The UI side relies on having the name of the owner to find the extension on
its linked list. If we start emiting signals before that has been acquired,
we will miss page created signals.

https://bugzilla.gnome.org/show_bug.cgi?id=727139
Add sanity check warning for finding extensions

That will help debugging future issues with extensions not being found.

https://bugzilla.gnome.org/show_bug.cgi?id=727139