scripts/functions: do not abort on failed svn

In case of 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 can't be downloaded; we
want to post-pone the check until we can extract the main archive.

So:
- try to retrieve the add-on
- 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>

# Options specific to crosstool-NG acting as a backend

config IS_A_BACKEND

    string

    option env="CT_IS_A_BACKEND"

config BACKEND

    bool

    default y if IS_A_BACKEND = "y" || IS_A_BACKEND = "Y"

config BACKEND_ARCH

    string

    option env="CT_BACKEND_ARCH"

config BACKEND_KERNEL

    string

    option env="CT_BACKEND_KERNEL"

config BACKEND_LIBC

    string

    option env="CT_BACKEND_LIBC"