support/download: return different exit codes for different failures

support/download: return different exit codes for different failures

Return different exit codes depending on the error that occured:

  0: no error (hash file missing, or all hashes match)
  1: unknown option
  2: hash file exists, but at least one hash in error
  3: hash file exists, but no hash for file to check
  4: hash file exists, but at least one hash type unknown

This will be used in a later patch to decide whether the downloaded file
should be kept or removed.

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