[bt][l2cap] ERTM TxEngine: add failure callback

After exhausting retries for the receiver-ready poll message, we'll
need to close the L2CAP channel. As a step towards that goal, add a
failure callback to the ERTM TxEngine. The creator of the Engine will
be responsible for closing the channel when this callback is invoked.

Bug: BT-442
Test: bt-host-unittests

Change-Id: Id462dea7556747c0e46ef20b71f33961b3e82fee