package: ignore failed downloads with IGNORE_ERRORS

When IGNORE_ERROR is set, also skip broken downloads, this is required
to make a full "make download" run even if some upstream sources are
unreachable.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40768 3c298f89-4303-0410-b956-a3cf2f4a3e73