Make `errFetchManifest` public
Make `errFetchManifest` public

* It's quite useful to have error types exported so that libraries
can avoid substring matchers

Signed-off-by: Claudia Beresford <80d58ba8cbfbf831cfa674a754c8439fd7d2f06e@pivotal.io>

containers/image,containers/image,masters-of-cats/image