After creating a new extension, we try to open the main source
file with the default handler, which fails when there is none.
But given that the extension was created successfully, don't treat
a missing handler as failure, and print the path to the new extension
instead.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/905
The gnome-extensions tool code is really independent from the rest of the
code base, and could be used either as part of the gnome-shell build or as
stand-alone project (for example for the extension-ci docker image).
We can actually support both cases by moving the code to a subproject.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/877