scripts/functions: do not abort on failed download

In case of glibc/eglibc, some add-ons that were previously external are
now internal (bundled with the main sources).

So we do not want to fail if an add-on tarball can't be downloaded; we
want to post-pone the check until we can extract the main archive.

So:
- try to download the tarball
- if it fails, print a warning instead of calling CT_Abort
- return 1

So, components that want to catch the error and want to handle it can,
while components that do not will gracefuly fail thanks to our catching
every errors.

Bonus: it works without changing any existing retrieval procedure! :-)

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

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