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/services/user/userimpl/user.go

@@ -6,7 +6,6 @@ import (
 	"fmt"
 	"time"
 
-	"github.com/grafana/grafana/pkg/models"
 	"github.com/grafana/grafana/pkg/services/accesscontrol"
 	"github.com/grafana/grafana/pkg/services/dashboards"
 	"github.com/grafana/grafana/pkg/services/org"
@@ -84,7 +83,7 @@ func (s *Service) Create(ctx context.Context, cmd *user.CreateUserCommand) (*use
 		Email: cmd.Email,
 	}
 	usr, err = s.store.Get(ctx, usr)
-	if err != nil && !errors.Is(err, models.ErrUserNotFound) {
+	if err != nil && !errors.Is(err, user.ErrUserNotFound) {
 		return usr, err
 	}