makes unexpected mountpoints exception more verbose

ot_admin/Classes/Controller/SiteController.php

@@ -2345,7 +2345,11 @@ class SiteController extends ActionController
 
             if ($row['uid']) {
                 if ((string)$rootUid != (string)$row['db_mountpoints']) {
-                    throw new \RuntimeException("The be_user named '" . $expectedUsername . "' has unexpected mounted website(s). Abort.");
+                    throw new \RuntimeException(
+                        "The be_user named '" . $expectedUsername .
+                        "' has unexpected mounted website(s) (expected: " . $rootUid .
+                        ", found: " . (string)$row['db_mountpoints'] . "). Abort."
+                    );
                 }
                 $adminUid = $row['uid'];
             }