scripts/download.pl: fail loudly if provided hash is unsupported

Currently, if the provided hash is unsupported (length different from 32
or 64 bytes), we happily download the requested file without any kind of
checksum verification.

This is quite dangerous and may provide a false sense of security, because
a single typo in the hash (e.g. one character deleted by mistake) may skip
checksum verification entirely.

Instead, fail immediately if we don't support the provided hash.
In particular, if an external package repository decides to change the
hash algorithm one day, we will now fail loudly instead of skipping
checksum verification without complaints.

Note: if some users of scripts/download.pl knowingly provide an empty hash
because they don't need checksum verification, this change will break
them.  This does not seem to be the case currently, but if this feature is
ever needed, an option should be added to download.pl instead of relying
on the hash being empty.

Fixes: eaa4eba10a89 ("scripts/download.pl: add SHA-256 support")

Signed-off-by: Baptiste Jonglez <git@bitsofnetworks.org>

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