support/download: quick fix to avoid breaking on custom toolchains

When the user selects a custom toolchain to be downloaded, there's no
hash for that toolchain, so the download fails, now that hashes are
mandatory.

Fix that by simply exiting as if there was no error, until we have a
better fix...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>