[kn-plugin-workflow] Improve error message when deploying without running cluster on local

[domhanak]

Goal
Improve the error message when running kn workflow deploy –namespace apache-10 --custom-generated-manifests-dir ./ on a project that has YAML workflow.

Reason
No cluster up and running on local, should be solved by our docs and prerequisites

Details

- ✅ Manifest 01-sonataflow_hello.yaml successfully deployed in namespace apache-10
error: unable to decode "workflow.sw.yaml": Object 'Kind' is missing in
 '{"description":"Description","id":"hello","name":"Hello World","specVersion":"0.8.0","start":"HelloWorld",
"states":[{"data":{"message":"Hello World"},"end":true,"name":"HelloWorld","type":"inject"}],"version":"1.0"}'
 has a errorError:
 ❌ ERROR: applying deploy: ❌ ERROR: failed to deploy manifest workflow.sw.yaml,  
❌ ERROR: failed to execute kubectl apply command for workflow.sw.yaml: exit status 1

Expected result
A message stating there is no cluster running to where to deploy.