support/download: always fail when there's no hash

At the time we introduced hashes, we did not want to be too harsh in the
beginning, and give people some time to adapt and accept the hashes. So
we so far only whined^Wwarned about a missing hash (when the .hash file
exists).

Some time has passed now, and people are still missing updating hashes
when bumping packages.

Let's make that warning a little bit more annoying...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>