config: seperate AssertValid and AssertStructValid

Added an error structure to make it possible to get the specifics of the failure.

system/update.go

@@ -61,7 +61,7 @@ func (uc Update) File() (*File, error) {
 	if config.IsZero(uc.Update) {
 		return nil, nil
 	}
-	if err := config.AssertValid(uc.Update); err != nil {
+	if err := config.AssertStructValid(uc.Update); err != nil {
 		return nil, err
 	}
 

system/update_test.go

@@ -17,7 +17,6 @@
 package system
 
 import (
-	"errors"
 	"io"
 	"reflect"
 	"strings"
@@ -101,7 +100,7 @@ func TestUpdateFile(t *testing.T) {
 		},
 		{
 			config: config.Update{RebootStrategy: "wizzlewazzle"},
-			err:    errors.New("invalid value \"wizzlewazzle\" for option \"RebootStrategy\" (valid options: \"best-effort,etcd-lock,reboot,false\")"),
+			err:    &config.ErrorValid{Value: "wizzlewazzle", Field: "RebootStrategy", Valid: []string{"best-effort", "etcd-lock", "reboot", "false"}},
 		},
 		{
 			config: config.Update{Group: "master", Server: "http://foo.com"},