gnmi: Add SubscribeErr as replacement to Subscribe
gnmi: Add SubscribeErr as replacement to Subscribe

SubscribeErr returns an error instead of writing one to a err
channel. This should make use of this function easier since the user
doesn't need to juggle two different channels. It also works better
with an errgroup.

Change-Id: I872c7e0752e34e28b0d1b05c5ed88fca4301d23e

aristanetworks/goarista,aristanetworks/goarista