`bst push` will report success even if the local artifacts don't exist (and therefore aren't pushed)

Summary

Similar to #501 (closed), BuildStream will currently happily print SUCCESS with bst push, even when it doesn't do anything.

In this case, BuildStream won't push a non-existent artifact, but it will succeed anyway. This seems wrong.

Steps to reproduce

1. Have a project with no cached artifacts
2. Run bst push on any element

What is the current bug behavior?

BuildStream will print SUCCESS doing nothing (all to-be-pushed elements will be "waiting").

What is the expected correct behavior?

BuildStream should probably print FAILED, and SKIPPED for any artifact it doesn't push.

Relevant logs and/or screenshots

Possible fixes

Other relevant information

• BuildStream version affected: /milestone %BuildStream_v1.2

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