In https://github.com/symfony/filesystem/blob/master/LockHandler.php the error_handler is deactivated from lines 70-82. Thus, if an error is thrown in line 85-86, the error-message is wrong.

Found in contao/core-bundle upon symlink generation: contao/core-bundle#845
The system tries to open a specific file in /tmp but has no read/write access due to malconfiguration (?). A Critical Exception is thrown, but the error-message does not fit to the error.