The less grunt runner silently ignore missing files and continue with the build[1]::
Running "less:production" (less) task
>> Destination css/searxng.min.css not written because no source files were found.
>> 1 stylesheet created.
>> 1 sourcemap created.
Add filter function that calls grunt.fail() if the scr file does not exists.
[1] https://github.com/searxng/searxng/pull/750#discussion_r784357031
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>