run: Print an error for runtimes that aren't installed
run: Print an error for runtimes that aren't installed

Currently if you pass flatpak-run an app that's not installed it prints
an error message to that effect, but if you pass it a runtime that's not
installed (and use the full ref format, e.g.
runtime/org.gnome.Sdk//master) it prints this message and crashes:

(flatpak run:15104): GLib-CRITICAL **: g_propagate_error: assertion 'src != NULL' failed
flatpak:ERROR:app/flatpak-main.c:394:flatpak_run: assertion failed: (success || error)

This commit makes flatpak print the appropriate error message, while
still printing an "app not found" error if the user didn't specify
whether the ref is an app or runtime.

GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,handsome-feng/flatpak,flatpak/flatpak,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,flatpak/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,GeorgesStavracas/flatpak,matthiasclasen/flatpak,flatpak/flatpak,handsome-feng/flatpak,flatpak/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,flatpak/flatpak,matthiasclasen/flatpak