Fix Targeted error for non-existing NS

[aufi]

Adding check to targeted gathering if the provided NS exists.

This should avoid errors caused by not existing NS when getting Plans/VMs in later parts of the targeted gathering script.

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1996360

[aufi]

Tested locally with forked-repo image: [must-gather-b9cqx] POD 2021-10-13T09:54:13.628043324Z ERROR: Specified NS doesn't exist, targeted gathering cannot...

$ oc adm must-gather --image=quay.io/maufart/forklift-must-gather:latest -- NS=ns1 PLAN=plan1 VM=vm-3345 /usr/bin/targeted
[must-gather      ] OUT Using must-gather plug-in image: quay.io/maufart/forklift-must-gather:latest
[must-gather      ] OUT namespace/openshift-must-gather-nmc7f created
[must-gather      ] OUT clusterrolebinding.rbac.authorization.k8s.io/must-gather-zckhd created
[must-gather      ] OUT pod for plug-in image quay.io/maufart/forklift-must-gather:latest created
[must-gather-b9cqx] POD 2021-10-13T09:54:13.387311810Z Targeted gathering for Forklift in namespaces [konveyor-forklift]
[must-gather-b9cqx] POD 2021-10-13T09:54:13.402848828Z Targeted gathering for Namespace: ns1
[must-gather-b9cqx] POD 2021-10-13T09:54:13.628043324Z ERROR: Specified NS doesn't exist, targeted gathering cannot continue. Set NS to an existing namespace.
[must-gather-b9cqx] OUT waiting for gather to complete
[must-gather-b9cqx] OUT downloading gather output
[must-gather-b9cqx] OUT receiving incremental file list
[must-gather-b9cqx] OUT ./
[must-gather-b9cqx] OUT 
[must-gather-b9cqx] OUT sent 27 bytes  received 40 bytes  44.67 bytes/sec
[must-gather-b9cqx] OUT total size is 0  speedup is 0.00
[must-gather      ] OUT clusterrolebinding.rbac.authorization.k8s.io/must-gather-zckhd deleted
[must-gather      ] OUT namespace/openshift-must-gather-nmc7f deleted