Remove auto-init behaviour

Summary

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.

Steps to reproduce

cd $(mktemp -d)
bst show no.such.bst

What is the current bug behavior?

Stops the user with a prompt.

What is the expected correct behavior?

We have now agreed on printing a hint, instead of stopping on a prompt.

Relevant logs and/or screenshots

Other relevant information

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

• BuildStream version affected: /milestone %BuildStream_v1.x