Avoid to print runtime error if kubectl context is not set

[chaozbj]

This PR is used to address issue #32, following kn/client, we will delay creating kubenets client interface till subcommand running.

Fixes #32

[codecov[bot]]

Codecov Report

Merging #40 (5a5ed94) into master (f4b8a49) will increase coverage by 0.49%. The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master      #40      +/-   ##
==========================================
+ Coverage   78.76%   79.26%   +0.49%     
==========================================
  Files          20       20              
  Lines        1003     1032      +29     
==========================================
+ Hits          790      818      +28     
+ Misses        156      155       -1     
- Partials       57       59       +2     

Impacted Files	Coverage Δ
core/root.go	60.00% <ø> (ø)
pkg/types.go	22.00% <18.18%> (+2.00%)	:arrow_up:
pkg/command/profiling/profiling.go	81.85% <84.61%> (-0.50%)	:arrow_down:
pkg/command/autoscaling/list.go	92.85% <100.00%> (+0.26%)	:arrow_up:
pkg/command/autoscaling/update.go	70.27% <100.00%> (+0.82%)	:arrow_up:
pkg/command/domain/list.go	85.18% <100.00%> (+1.85%)	:arrow_up:
pkg/command/domain/set.go	96.49% <100.00%> (+0.19%)	:arrow_up:
pkg/command/domain/unset.go	94.11% <100.00%> (+0.56%)	:arrow_up:
pkg/command/registry/add.go	92.94% <100.00%> (+0.25%)	:arrow_up:
pkg/command/registry/list.go	87.93% <100.00%> (+0.65%)	:arrow_up:
... and 1 more

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[chaozbj]

@rhuss @zhanggbj @lanceliuu Please help me to review, thanks a lot! @lanceliuu In types.go file, I used the

return err

to replace your codes when fail to check installation method:

fmt.Println("failed to get installation method:", err)
os.Exit(1)

Because the os.Exit(1) will break some UT. I tried to keep them and find some ways to make UT pass, but failed. If the installation method is not necessary for every subcommand, I would like to recommend to delay getting it till the subcommand really needs it.

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chaozbj, maximilien, zhanggbj

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/knative-sandbox/kn-plugin-admin/blob/master/OWNERS)~~ [maximilien,zhanggbj] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[zhanggbj]

/lgtm /assign @maximilien

Looks great! Thanks for fixing the issue and especially improving test coverage. Merging it!