registry: allow fallback on unknown errors
registry: allow fallback on unknown errors

This patch fixes a bug where a user specifies a v1 mirror for
--registry-mirror and pull an image from the Hub.

It used to not fallback because of an unexpected error returned when
trying to JSON marshal nginx output.

We now ensure that any unexpected error falls back to the next endpoint
in the list.

Signed-off-by: Tibor Vass <3f1326dff4d85ad4ec938fb33f64575ace96db26@docker.com>

thaJeztah/distribution,thaJeztah/distribution