Authn: Prevent empty username and email during sync (#76330)

* Move errors to error file

* Move check for both empty username and email to user service

* Move check for empty email and username to user service Update

* Wrap inner error

* Set username in test

pkg/services/user/model.go

@@ -1,7 +1,6 @@
 package user
 
 import (
-	"errors"
 	"fmt"
 	"strings"
 	"time"
@@ -20,18 +19,6 @@ const (
 	HelpFlagDashboardHelp1
 )
 
-// Typed errors
-var (
-	ErrCaseInsensitive   = errors.New("case insensitive conflict")
-	ErrUserNotFound      = errors.New("user not found")
-	ErrUserAlreadyExists = errors.New("user already exists")
-	ErrLastGrafanaAdmin  = errors.New("cannot remove last grafana admin")
-	ErrProtectedUser     = errors.New("cannot adopt protected user")
-	ErrNoUniqueID        = errors.New("identifying id not found")
-	ErrLastSeenUpToDate  = errors.New("last seen is already up to date")
-	ErrUpdateInvalidID   = errors.New("unable to update invalid id")
-)
-
 type User struct {
 	ID            int64 `xorm:"pk autoincr 'id'"`
 	Version       int