scripts/feeds: return error status from feed update

This patch is a modified version of the patch being discussed at here:
https://patchwork.ozlabs.org/patch/471303/


from Martin Strbacka <martin.strbacka@nic.cz>

My version modifies scripts/feeds so that an error with one of the feeds just
raises the error flag, but script continues and tries to update the other
feeds. After all feeds have been updated, the script returns 1 if at least
one feed failed, and 0 on success with all feeds. The user can then utilise
the status in his build script, if he wants.

signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>

SVN-Revision: 46374