Add Namespace Check for Task Describe Command

[danielhelfand]

Following similar approaches outlined in #350 and #349, this pull request is part of addressing #311. An error message has been added for tkn task desc when a namespace doesn't exist on a cluster.

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
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• Includes tests (if functionality changed/added)
• Includes docs (if user facing)
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Release Notes

Adds error message to tkn task describe when a namespace does not exist

[tekton-robot]

Hi @danielhelfand. Thanks for your PR.

I'm waiting for a tektoncd member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[piyush-garg]

/ok-to-test

[tekton-robot]

The following is the coverage report on pkg/.
Say /test pull-tekton-cli-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/cmd/task/describe.go	92.5%	91.1%	-1.4

[sthaha]

/lgtm

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: sthaha

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