Organize server pre-func logic in middlewares.

It defines global middlewares for every request.
This makes the server slightly more composable.

Signed-off-by: David Calavera <david.calavera@gmail.com>

errors/server.go

@@ -0,0 +1,27 @@
+package errors
+
+import (
+	"net/http"
+
+	"github.com/docker/distribution/registry/api/errcode"
+)
+
+var (
+	// ErrorCodeNewerClientVersion is generated when a request from a client
+	// specifies a higher version than the server supports.
+	ErrorCodeNewerClientVersion = errcode.Register(errGroup, errcode.ErrorDescriptor{
+		Value:          "NEWERCLIENTVERSION",
+		Message:        "client is newer than server (client API version: %s, server API version: %s)",
+		Description:    "The client version is higher than the server version",
+		HTTPStatusCode: http.StatusBadRequest,
+	})
+
+	// ErrorCodeOldClientVersion is generated when a request from a client
+	// specifies a version lower than the minimum version supported by the server.
+	ErrorCodeOldClientVersion = errcode.Register(errGroup, errcode.ErrorDescriptor{
+		Value:          "OLDCLIENTVERSION",
+		Message:        "client version %s is too old. Minimum supported API version is %s, please upgrade your client to a newer version",
+		Description:    "The client version is too old for the server",
+		HTTPStatusCode: http.StatusBadRequest,
+	})
+)