Add Namespace Check to TaskRun Describe Command

[danielhelfand]

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

• Includes tests (if functionality changed/added)
• Includes docs (if user facing)
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Release Notes

Adds error message to tkn taskrun 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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[piyush-garg]

/ok-to-test

[piyush-garg]

/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/taskrun/describe.go	85.7%	84.8%	-0.9

[piyush-garg]

/test pull-tekton-cli-unit-tests

[vdemeester]

/lgtm

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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