fix(upgrade): report template read failures accurately

Use the upgrade file-access error when mainfile.dist.php cannot be read in Upgrade_230::write_mainfile() instead of reporting a mainfile write failure.

This makes upgrader diagnostics clearer when the template file is missing or unreadable.

Author: mambax7

upgrade/upd-2.0.18-to-2.3.0/index.php

@@ -418,7 +418,7 @@ public function write_mainfile($vars)
 
         $lines = file($file);
         if (false === $lines) {
-            echo ERR_COULD_NOT_WRITE_MAINFILE;
+            echo sprintf(defined('_FILE_ACCESS_ERROR') ? _FILE_ACCESS_ERROR : 'Could not access the file %s', basename($file));
 
             return false;
         }