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 <tibor@docker.com>
(cherry picked from commit a21ba12f4e7f11c17e6d665716f3060b9923b11c)

registry/registry.go

@@ -17,6 +17,7 @@ import (
 	"github.com/Sirupsen/logrus"
 	"github.com/docker/distribution/registry/api/errcode"
 	"github.com/docker/distribution/registry/api/v2"
+	"github.com/docker/distribution/registry/client"
 	"github.com/docker/distribution/registry/client/transport"
 	"github.com/docker/docker/autogen/dockerversion"
 	"github.com/docker/docker/pkg/parsers/kernel"
@@ -211,8 +212,14 @@ func ContinueOnError(err error) bool {
 		return ContinueOnError(v.Err)
 	case errcode.Error:
 		return shouldV2Fallback(v)
+	case *client.UnexpectedHTTPResponseError:
+		return true
 	}
-	return false
+	// let's be nice and fallback if the error is a completely
+	// unexpected one.
+	// If new errors have to be handled in some way, please
+	// add them to the switch above.
+	return true
 }
 
 // NewTransport returns a new HTTP transport. If tlsConfig is nil, it uses the