Additional files are dropped if original tar file can't be created

[fginther]

collect-logs allows additional files to specified that will be added to the generated tar file. However, if that file isn't created (because no juju model could be found) the additional files aren't added either and the result is no log file is created.

The desired outcome is to create a log file as requested with just the files specified on the command line.

This is what the failure looks like:

autopilot-log-collector/collect-logs /tmp/jenkins/workspace/landscape-system-tests/build-area/artifacts/all-logs.tar.gz system-tests-console.log juju-status.json
ERROR, no units found. Make sure the right juju environmentis set.