meta-launcher: g_propagate_*_error takes ownership

This means we need to make sure we don't accidentally free the provided source GError (which automatically happens with g_autoptr), so use g_steal_pointer().

This fixes an issue where, when launched in a bubblewrap environment (such as the one provided by Buildstream), we would get the following warning message:

mutter-WARNING **: 8:31:35:069: Can't initialize KMS backend: (null)

... which isn't that useful when trying to debug the actual issue.