Clarify repository naming constraints for registry API

After discussion, it was found that one of the proposed regular expressions
incorrectly limited separator delimited compoonents to two characters. The
desired restriction is to have repository name components limited to two
characters minimum. This changeset accomplishes this by wrapping the regular
expressions in a validation function, returning detailed feedback on the
validation error.

With this change, the repository name regular expressions are no longer enough
to respond with 404s on invalid repo names. Changes to the router will need to
be added to support this.