Fixes issue #740

Shell --build has optional buildtrees

Closes #740

See merge request !986

README.rst: Add license badge

See merge request !1014

Angelos Evripiotis authored Dec 19, 2018 and Angelos Evripiotis committed Dec 19, 2018

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

Fixes #826