Fix issue with terminationTracker in remote dev module

Issue: gitlab-org/gitlab#411205 (closed)

What does this MR do

• Fixes issue in the termination tracker that causes the termination of non-existing workspaces to be hung and remain unresolved. Attempting to terminate such workspaces results in case states of such workspaces being stuck at Terminating. This issue occurs because earlier implementation of the termination tracker only tracked workspaces that exist in the cluster. Updates have been made to accommodate the possibility of non-existing workspaces in the logic.
• Update k8s client to better distinguish between cases where an error is returned vs when a namespace is not found
• Add specs to verify the fix
• Changes have been verified locally

Signed-off-by: Hunar Khanna hkhanna@gitlab.com