Include unrecognized builder in error

[jonjohnsonjr]

Description

Improve the skaffold init --artifact error mesage for unrecognized builders.

User facing changes

Different error message.

Before

$ skaffold init --artifact '{"builder": "404"}'
FATA[0001] processing cli artifacts: unknown builder type in CLI artifacts 

After

$ skaffold init --artifact '{"builder": "404"}'
FATA[0001] processing cli artifacts: unknown builder type in CLI artifacts: "404" 

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes unit tests
• Mentions any output changes.
• Adds documentation as needed: user docs, YAML reference, CLI reference.
• Adds integration tests if needed.

Reviewer Notes

• The code flow looks good.
• Unit test added.
• User facing changes look good.

[codecov]

Codecov Report

Merging #3595 into master will increase coverage by 0.01%.
The diff coverage is 100%.

Impacted Files	Coverage Δ
pkg/skaffold/initializer/builders.go	94.2% <100%> (ø)	⬆️
...affold/kubernetes/portforward/kubectl_forwarder.go	68.29% <0%> (+2.43%)	⬆️

[balopat]

Thank you! LGTM.