BREAK: remove auto-init behaviour

Description

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 c1f571f4

Changes proposed in this merge request:

• Remove auto-init and corresponding prompt config.
• Update NEWS.

Fixes #826 (closed)

---