bst artifact pull: 'artifact not found' is success

Summary

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

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 <<EOF

artifacts:
    url: http://localhost:8888
    push: false
EOF

bst artifact pull hello.bst

What is the current bug behavior?

(bst) running-commands$ bst artifact pull hello.bst
[--:--:--][5a1a024a][ pull:hello.bst                     ] START   running-commands/hello/5a1a024a-pull.84519.log
[--:--:--][5a1a024a][ pull:hello.bst                     ] STATUS  Pulling artifact 5a1a024a <- http://localhost:8888
[--:--:--][5a1a024a][ pull:hello.bst                     ] INFO    Remote (http://localhost:8888) does not have 5a1a024a cached
[00:00:00][5a1a024a][ pull:hello.bst                     ] SKIPPED Pull
[00:00:00][        ][ main:core activity                 ] SUCCESS Pull

Pipeline Summary
  Total:      3
  Session:    1
  Pull Queue: processed 0, skipped 1, failed 0

(bst) running-commands$ echo $?
0

What is the expected correct behavior?

Something more like:

(bst) running-commands$ bst artifact pull hello.bst
[--:--:--][5a1a024a][ pull:hello.bst                     ] START   running-commands/hello/5a1a024a-pull.84519.log
[--:--:--][5a1a024a][ pull:hello.bst                     ] STATUS  Pulling artifact 5a1a024a <- http://localhost:8888
[--:--:--][5a1a024a][ pull:hello.bst                     ] INFO    Remote (http://localhost:8888) does not have 5a1a024a cached
[00:00:00][5a1a024a][ pull:hello.bst                     ] SKIPPED Pull
[00:00:00][        ][ main:core activity                 ] FAILURE Pull

Pipeline Summary
  Total:      3
  Session:    1
  Pull Queue: processed 0, skipped 0, failed 1

Remote (http://localhost:8888) does not have 5a1a024a (hello.bst) cached

(bst) running-commands$ echo $?
1

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