Common Way of Referring to Tekton Resources for User Facing Messages: Task

[danielhelfand]

Part of #605

This pull request changes all user-facing messages to use Task and TaskRun instead of a variety of ways it is referenced throughout tkn (e.g., taskrun, TaskRun, etc.).

This pull request only focuses on tkn task commands, but there are other Task/TaskRun references throughout tkn that should be updated as well, but will be addressed in pull requests pertaining to the commands or helper packages where TaskRun is referenced.

Additionally, this pr removes printing error messages to Stderr, and instead returns the errors to be handled by Cobra instead.

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)
• Run the code checkers with make check
• Regenerate the manpages, docs and go formatting with make generated
• Commit messages follow commit message best practices

Release Notes

Standardize the use of Task/TaskRun in user-facing messages for tkn task commands

[tekton-robot]

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

File	Old Coverage	New Coverage	Delta
pkg/cmd/task/describe.go	73.8%	75.4%	1.6
pkg/cmd/task/list.go	78.4%	82.9%	4.5
pkg/cmd/task/logs.go	75.3%	73.2%	-2.1

[vdemeester]

/meow

[tekton-robot]

@vdemeester:

In response to this:

/meow

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[pradeepitm12]

/lgtm

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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