flatpak
Don't ignore `--build-shell` option when configuration fails
--build-shell is supposed to help in debbuging problems for modules, that may be a failure to build, to configure, etc.
So it doesn't make much sense that if the configuration step fails we ignore the --build-shell option, which the developer is probably using it to debug that same configure problem.
Fixes #479
PR #508 tried to fix this bug, but @TingPing suggested there in a comment to better use another approach which I implemented in this PR.
https://github.com/flatpak/flatpak-builder/pull/508 seems like the better fix to me than this.
It does change the behavior since previously it would configure the project, then open a shell, now it just opens a shell.
I'm not sure why opening the shell first would be a problem instead of ignoring errors in build-shell.
If you are debugging a build which is broken in the configure step, you likely want to open a shell first and then manually run the configure commands etc. to debug it?
