assets: error logging if binary are missing

* Adding a check for LESS_BIN and CLEANCSS_BIN presence at startup.
  It switches to ASSETS_DEBUG=True if you ignore the error message.

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>
Reported-by: Esteban J. G. Gabancho <esteban.gabancho@gmail.com>