feat(routes): better error message in case of missing annotations (#1150)

putting this info into error detail would be ideal, but skopeo
doesn't print them, so overwrite the error message.

Signed-off-by: Petu Eusebiu <peusebiu@cisco.com>

pkg/api/errors.go

@@ -11,6 +11,12 @@ type Error struct {
 	Detail      interface{} `json:"detail,omitempty"`
 }
 
+func (e Error) WithMessage(msg string) Error {
+	e.Message = msg
+
+	return e
+}
+
 type ErrorList struct {
 	Errors []*Error `json:"errors"`
 }