Misleading SUCCESS status when artifact push is skipped

Summary

The bst push command provides misleading info text regarding the status of the pushing of an artifact.

Steps to reproduce

• Set up a remote artifact server (I had mine set up locally using http)
  • bst-artifact-server --port 11001 --enable-push artifacts
• Set up a simple buildstream project and add an artifact cache as instructed here
• Attempt to bst push <element>

What is the current bug behavior?

On skipping of caching an artifact, it is reported as SUCCESS as shown on line 37 of [1]. However at the bottom is does clearly represent that the artifact was skipped but this would be an issue if individual artifacts were skipped in a large collection.

What is the expected correct behavior?

It should be made more clear to the user that the artifact was skipped during caching. e.g. SKIPPED Push

Relevant logs and/or screenshots

[1] : Paste

Possible fixes

job.py: Adapt Job._child_action as it currently displays anything which isn't an exception as a success.

Other relevant information

• BuildStream version affected: BuildStream_v1.1.4

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