fix(core): generic solution for issue #111 for all command groups

[maximilien]

BFS exploration of all sub-commands from rootCmd and adds a RunE for all commands that are command groups with child sub-commands and without a RunE.

The added RunE will return the correct errors when the command is called with empty sub-command or with an unknown sub-command. It will also print the command help message first.

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I did not add a test for this since the we need to decide on #185 and #214 and #134. Once we do I will add the gotest.tools or Ginkgo test for it.

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[maximilien]

/assign @cppforlife

cc: @rhuss

[navidshaikh]

Works better than v1 fix of the issue.
@maximilien :

1. Can we separate the help section and error message printed with a line please? helps make error message more visible / readable.
2. I tested PR by running ./hack/build.sh, its reporting updates to command docs files docs/cmd/kn_revision.md and docs/cmd/kn_service.md as well, lets also include that with the PR.
3. Can we add some tests please?

[rhuss]

3\. Can we add some tests please?

I think @maximilien wants just to wait with tests until with which testing setup we want to continue. So let's wait until tomorrows WG meeting.

[maximilien]

OK @rhuss, I think I am convinced gotest.tools is better for this project. You were right from the start...

I added a test for root.go that also tests the functionality for issue #111 and I like the result. See latest update here.

cc: @navidshaikh and @cppforlife

[rhuss]

@maximilien I think you missed to check in the vendored files (use a build.sh -u to populate the vendor dir)

The PR looks good to me, let's merge it tomorrow if there are no objections against gotest.tools in today's WG call.

[maximilien]

@maximilien I think you missed to check in the vendored files (use a build.sh -u to populate the vendor dir)

The PR looks good to me, let's merge it tomorrow if there are no objections against gotest.tools in today's WG call.

Yes, done, thanks @rhuss, one day I will remember to do this before pushing :) Converting all ginkgo tests now. Best.

[knative-metrics-robot]

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

File	Old Coverage	New Coverage	Delta
pkg/kn/core/root.go	Do not exist	52.5%

[rhuss]

/lgtm

[rhuss]

@maximilien cool, thanks !

Now that gotest.tools is in, we have to rebase our other PRs which introduce them (but should be hopefully trivial)