Improve error handling in localSchemasClient

[eddycharly]

We should check directories exist and return an error if reading the directory returns an error in:

kubectl-validate/pkg/openapiclient/local_schemas.go

Line 35 in 76a49ca

apiGroups, _ := utils.ReadDir(k.fs, filepath.Join(k.dir, "apis"))

kubectl-validate/pkg/openapiclient/local_schemas.go

Line 51 in 76a49ca

coregroup, _ := utils.ReadDir(k.fs, filepath.Join(k.dir, "api"))

[hiroyaonoe]

I would like to work on this issue.
Do I simply return the error that ReadDir returns (with an additional message)?
Or should I do some kind of error handling?
/assign