[fix] stop less grunt runner on missing files

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>

0 files changed, 0 insertions, 0 deletions