Add Namespace Check to PipelineRun Logs 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 pr logs 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.
• Run the code checkers with make check
• Commit messages follow commit message best practices

Release Notes

Adds error message to tkn pr logs command when a namespace does not exist

[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/pipelinerun/logs.go	92.0%	92.6%	0.6

[vdemeester]

/lgtm
/meow

[tekton-robot]

@vdemeester:

In response to this:

/lgtm
/meow

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[tekton-robot]

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