Add Namespace Check to Pipeline Describe Command

[danielhelfand]

Following similar approaches outlined for tkn commands, this pull request is part of addressing #311. An error message has been added for tkn pipeline describe 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
review them:

• Includes tests (if functionality changed/added)
• Includes docs (if user facing)
• Regenerate the manpages and docs with make docs and make man if needed.
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Release Notes

Adds error message to tkn pipeline 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.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[chmouel]

/ok-to-test

[chmouel]

/lgtm

[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/pipeline/describe.go	90.9%	91.4%	0.5

[piyush-garg]

/lgtm

[chmouel]

/approve

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chmouel, piyush-garg

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