Throw an error if registries folder does not exist

[jalvarezit]

Motivation

auth-oci/io.go

Line 18 in c95d1b6

filepath.Walk(dir, func(path string, info os.FileInfo, err error) error {

If there is no registries directory it ends up in a empty list throwing a misleading error:

auth-oci/io.go

Line 69 in c95d1b6

panic(fmt.Sprintf("Registry %s not found", url))

Acceptance criteria

A more descriptive error should be thrown