Fix `paver update_assets` warning on missing sass dir (#25771)

On missing sass directory, the following kind of warning was printed:

        Sass dir '[{'sass_source_dir': path('lms/static/sass'), 'css_destination_dir': Path('/openedx/themes/indigo/lms/static/css'), 'lookup_paths': [Path('/openedx/themes/indigo/lms/static/sas│..  s/partials'), path('lms/static/sass/partials'), path('lms/static/sass')]}, {'sass_source_dir': Path('/openedx/themes/indigo/lms/static/sass'), 'css_destination_dir': Path('/openedx/themes│.  /indigo/lms/static/css'), 'lookup_paths': [Path('/openedx/themes/indigo/lms/static/sass/partials'), path('lms/static/sass/partials'), path('lms/static/sass')]}, {'sass_source_dir': Path('│ /openedx/themes/indigo/lms/static/certificates/sass'), 'css_destination_dir': Path('/openedx/themes/indigo/lms/static/certificates/css'), 'lookup_paths': [Path('/openedx/themes/indigo/lms│ /static/sass/partials'), Path('/openedx/themes/indigo/lms/static/sass')]}]' does not exists, skipping sass compilation for '/openedx/themes/indigo'

Which was rather inconvenient. Instead, we now print the following
warning:

    Sass dir '/openedx/themes/indigo/lms/static/certificates/sass' does not exists, skipping sass compilation for '/openedx/themes/indigo'