bst artifact pull: 'artifact not found' is success

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/970)
In GitLab by [[Gitlab user @aevri]](https://gitlab.com/aevri) on Mar 21, 2019, 17:50

## Summary

[//]: # (Summarize the bug encountered concisely)

If you try to `bst artifact pull` an artifact that the server does not have, the command exits with code `0` and prints 'SUCCESS'.

It seems that it should exit with non-zero and print less positive text about the operation.

## Steps to reproduce

[//]: # (How one can reproduce the issue - this is very important)

```bash
git clone https://gitlab.com/buildstream/buildstream.git
cd buildstream/doc/examples/running-commands

REPO=$(mktemp -d)
bst-artifact-server "$REPO" -p 8888 &

cat >>project.conf <

Start in doc/examples/running-commands and reproduce the behavior with bst artifact pull hello.bst against an empty bst-artifact-server repository. Trace how the skipped pull affects the final pipeline summary and exit code; done means a missing artifact reports failure, includes the artifact context, and exits non-zero as shown in the expected output.

Written by the indexing model from the issue text.