_artifactcache/ostreecache.py: Fix artifact initialization result tuple

Antoine Wacheuxrequested to merge
bloomberg/bst/buildstream:awacheux/artifact-type-error into master

Description

Prevent BuildStream from crashing when a remote OSTree artifact cache can't be properly initialized.

Changes proposed in this merge request:

  • Fix the result tuple in case of initialization failure

This merge request, when approved, will close issue/bug:

CHANGELOG/Release Notes

  • Fix the crash caused when a remote OSTree artifact cache can't be properly initialized.
Edited by Antoine Wacheux

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