upgrade-manager-v2: Throw error and fail the CR when Scaling Group is not found.

[shreyas-badiger]

Is this a BUG REPORT or FEATURE REQUEST?:
Bug report
What happened:
I submitted a rollup CR with a non-existent scaling group.
What you expected to happen:
The CR must fail and throw an error.
How to reproduce it (as minimally and precisely as possible):
Submit a rollupCR with an invalid scaling group name.

Environment:

• rolling-upgrade-controller version 2
• Kubernetes version : 1.18