Improve error message when dependent refs are not installed

[cpba]

At the moment, flatpak-builder will download the required sdk if --install-deps-from=REMOTE is passed.

It would be a nice feature for beginners if flatpak-builder were to try downloading the sdk from the installed remotes when this isn't passed. Most of the time a beginner will only have one remote and this would ease some of the initial pain when learning to use the tool.