Export "no basic auth credentials" as an error value
Export "no basic auth credentials" as an error value

Making this an exported error value will allow users of the
registry/client/auth module to have consistent behavior between
authentication failures and cases where no credentials are provided.

Signed-off-by: Aaron Lehmann <8ecfc6017a87905413dcd7d63696a2a4c351b604@docker.com>

thaJeztah/distribution,thaJeztah/distribution