mesh: Fix handling of failed transmissions
mesh: Fix handling of failed transmissions

When sending a segmented message, the state could get stuck if the
advertising bearer fails in transmitting and we don't detect that it
happened. Add a send_start callback for all packets so we can always
know if sending fails.

X-Original-Commit: c2f6fa5baf8eab609a705347e1c1b881e48bd710

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble