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>

 This is the README for crosstool-NG

 Crosstool-NG follows the autoconf dance. So, to get you

 kick-started, just run:

   ./configure --help

 You will find the documentation in the directory 'docs'.

 Here is a quick overview of what you'll find there:

   0 - Table of content

   1 - Introduction

   2 - Installing crosstool-NG

   3 - Configuring a toolchain

   4 - Building the toolchain

   5 - Using the toolchain

   6 - Toolchain types

   7 - Contributing

   8 - Internals

   A - Credits

   B - Known issues

   C - Misc. tutorials

 You can also point your browser at:

   http://ymorin.is-a-geek.org/projects/crosstool

 Aloha!