BREAK: remove auto-init behaviour

In the event that the project could not be found, stop BuildStream from
asking if the user would like to create a new project. Exit with error
instead, and give a hint to the user in case they're new.

As proposed on the mailing list here:
https://mail.gnome.org/archives/buildstream-list/2018-December/msg00082.html

The new interaction looks like this:

    $ bst show nonsuch.bst
    No project found. You can create a new project like so:

        bst init

    Error loading project: None of ['project.conf', '.bstproject.yaml']
    found in '/src/temp/blah' or any of its parent directories

Fixes #826