You probably want your link tag to say:
<link rel="stylesheet/less" type="text/css" href="404.less">
However, if this is being served from a web server, it may happen that it detects the wrong mime type from the file extension. If it is an Apache server, then there is an easy fix. Add this / create the .htaccess file to your webroot containing the line:
AddType text/css less
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