Add user-friendly validation of builder/artifact compatibility

[tstromberg]

Fixes #4161

Example case

If cluster is removed from examples/kaniko/skaffold.yaml, and skaffold build is run.

Old error message

couldn't build "skaffold-example": undefined artifact type: {DockerArtifact:<nil> BazelArtifact:<nil> JibArtifact:<nil> KanikoArtifact:0xc000542be0 BuildpackArtifact:<nil> CustomArtifact:<nil>}

New error message

invalid skaffold config: Found a 'kaniko' artifact, which is incompatible with the 'local' builder:

image: skaffold-example
context: .
kaniko:
  dockerfile: Dockerfile
  initImage: busybox
  image: gcr.io/kaniko-project/executor:v0.20.0@sha256:f9a4a760166682c7c7aeda3cc263570682e00848ab47737ed8ffcc3abd2da6c3
  cache: {}

To use the 'kaniko' builder, add the 'cluster' stanza to the 'build' section of your configuration. For information, see https://skaffold.dev/docs/pipeline-stages/builders/

[codecov]

Codecov Report

Merging #4312 into master will increase coverage by 0.04%.
The diff coverage is 65.71%.

@@            Coverage Diff             @@
##           master    #4312      +/-   ##
==========================================
+ Coverage   71.96%   72.00%   +0.04%     
==========================================
  Files         323      324       +1     
  Lines       12370    12404      +34     
==========================================
+ Hits         8902     8932      +30     
- Misses       2904     2905       +1     
- Partials      564      567       +3     

Impacted Files	Coverage Δ
pkg/skaffold/build/cluster/cluster.go	27.27% <0.00%> (ø)
pkg/skaffold/build/dependencies.go	0.00% <0.00%> (ø)
pkg/skaffold/build/local/local.go	62.79% <40.00%> (ø)
pkg/skaffold/schema/validation/validation.go	94.95% <64.70%> (-5.05%)	⬇️
pkg/skaffold/build/misc/artifact_type.go	90.47% <90.47%> (ø)
pkg/skaffold/build/gcb/spec.go	100.00% <100.00%> (+5.88%)	⬆️
...affold/kubernetes/portforward/kubectl_forwarder.go	63.41% <0.00%> (+2.43%)	⬆️

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[dgageot]

Would it make sense to move this validation to validation.Process()?

[tstromberg]

@dgageot - That seems reasonable. I wasn't aware of the validation package function before, but I could easily move the user-friendly error message there, and leave the internal error where it is now.

[dgageot]

@tstromberg I think that would make sense

[tstromberg]

Done. Please take another look.

[nkubala]

nice, these error messages are way better. small nits but otherwise LGTM