Description of problem:
Run the `oc adm must-gather` command with a cluster has issue, will hit error: 
WARNING: cannot use rsync: rsync not available in container
WARNING: cannot use tar: tar not available in container
[must-gather-z48bh] OUT gather output not downloaded: No available strategies to copy.
[must-gather-z48bh] OUT 
[must-gather      ] OUT clusterrolebinding.rbac.authorization.k8s.io/must-gather-m5q2b deleted
[must-gather      ] OUT namespace/openshift-must-gather-czn2b deleted
error: unable to download output from pod must-gather-z48bh: No available strategies to copy.
We can sure we use the default image of must-gather and can't reproduce this on a normal cluster. 

Version-Release number of selected component (if applicable):
[root@dhcp-140-138 roottest]#  oc version --client
Client Version: 4.7.0-0.nightly-2021-01-19-014259


How reproducible:
always

Steps to Reproduce:
1. The cluster has some issue, try to use `oc adm must-gather` to collect the cluster info. 
2.
3.

Actual results:
1. The `oc adm must-gather` failed with error, but the error info is not appropriate:   error: unable to download output from pod must-gather-z48bh: No available strategies to copy.
[root@dhcp-140-138 roottest]#  oc adm must-gather 
[must-gather      ] OUT Using must-gather plug-in image: quay.io/openshift-release-dev/ocp-v4.0-art-dev@sha256:c776ff6ca316d40b1369fa01a88cf7ddebc5afcec111268ef77ec775b52fe784
[must-gather      ] OUT namespace/openshift-must-gather-czn2b created
....

WARNING: cannot use rsync: rsync not available in container
WARNING: cannot use tar: tar not available in container
[must-gather-z48bh] OUT gather output not downloaded: No available strategies to copy.
[must-gather-z48bh] OUT 
[must-gather      ] OUT clusterrolebinding.rbac.authorization.k8s.io/must-gather-m5q2b deleted
[must-gather      ] OUT namespace/openshift-must-gather-czn2b deleted
error: unable to download output from pod must-gather-z48bh: No available strategies to copy.

Expected results:
1. Show definitely improve messaging when `oc adm must-gather` failed.

Additional info: