Workspace integration tests: Improve message when preview env build job fails

[mads-hartmann]

Description

This PR:

• Improves the error message when the integration tests fail due to a failed build for the preview environment (see example here - in 'How to test' you can see how I got the build job to fail to trigger this error case)
• Adds a clarify comment that explains why you see two Werft jobs for the integration test brach.
• I wasn't able to reproduce the dual slack messages that Kyle reported for job gitpod-custom-main.132 (here and here). I thought it might be due to the cleanup handler being invoked multiple times so I added a bit of logging so we can see if that's the case the next time.

Related Issue(s)

Fixes #11979

How to test

To trigger the scenario where the build job fails for the preview environment I manually killed the job while the integration test was running:

kubectl --context=dev -n werft delete pod gitpod-build-wk-inte-test-20220812064312.0

See the logs here

Release Notes

NONE

Documentation

N/A

Werft options:

• /werft with-preview