net: Improve error handling

net: Improve error handling

Take a pass at plumbing errors through to the users of the network stack

Currently only the start() function errors will be returned from
NetLoop(). recv() tends not to have errors, so that is likely not worth
adding. send() certainly can return errors, but this patch does not
attempt to plumb them yet. halt() is not expected to error.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Simon Glass <sjg@chromium.org>