buildstream should exit inmediatelly if the format is not supported

This was reported in IRC:

$:~/bst_projects/gnome-build-meta$ bst build --track-all core/gedit.bst
Error loading project: /home/jamesennis/bst_projects/gnome-build-meta/project.conf [line 11 column 13]: Unexpected key: ref-storage

* Then I commented out ref-storage: *

$:~/bst_projects/gnome-build-meta$ bst build --track-all core/gedit.bst
Error loading project: Project requested format version 5, but BuildStream 1.1 only supports up until format version 4

Edited Mar 23, 2018 by Javier Jardón

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