Chore: Move user errors to user service (#52460)

* Move user not found err to user service

* User ErrCaseInsensitive from user pkg

* User ErrUserAlreadyExists from user pkg

* User ErrLastGrafanaAdmin from user pkg

* Remove errors from model

pkg/models/user.go

@@ -1,21 +1,11 @@
 package models
 
 import (
-	"errors"
 	"time"
 
 	"github.com/grafana/grafana/pkg/services/user"
 )
 
-// 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")
-)
-
 type Password string
 
 func (p Password) IsWeak() bool {