Plugin handling if it retuns with non-zero exit status

[navidshaikh]

Bug report

If a plugin exits with non-zero status, help usage hint is printed, which seems to be unnecessary.

Expected behavior

If plugin execution path is invoked, let plugin print message or error.

Steps to reproduce the problem

1. Create a sample plugin

cat ~/.config/kn/plugins/kn-source-test.sh 
#!/bin/bash
echo "hello"
exit 1

2. Make sure its executable

chmod +x ~/.config/kn/plugins/kn-source-test.sh

3. Run

kn source test.sh
hello
Error: exit status 1
Run 'kn --help' for usage

Notice the last help usage message printed.

kn version

master

kn version 
Version:      v20200619-local-a98d22a1-dirty
Build Date:   2020-06-19 15:35:19
Git Revision: a98d22a1
Supported APIs:
* Serving
  - serving.knative.dev/v1 (knative-serving v0.15.0)
* Eventing
  - sources.knative.dev/v1alpha2 (knative-eventing v0.15.0)
  - eventing.knative.dev/v1alpha1 (knative-eventing v0.15.0)

/kind cleanup

[danielhelfand]

/assign