Improve error msg returned when no kustomization file is found

[alexbrand]

Initial attempt at improving the error message so that the user knows which files are supported by kustomize.

Very much open to feedback on wording, structure, etc.

Fixes #770

[Liujingfang1]

@alexbrand go fmt failed in ci.

pkg/target/kusttarget.go:91: line is 185 characters (lll)
		return nil, fmt.Errorf("No kustomization file found in %s. Kustomize supports the following kustomization files: %s", ldr.Root(), strings.Join(constants.KustomizationFileNames, ", "))

[alexbrand]

Just updated to fix the line length. Thanks!

[Liujingfang1]

/lgtm

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