xud does not shutdown gracefully after failed swaps

[erkarl]

Steps to reproduce:

1. create xud-bob and xud-alice environments with funded ETH and BTC channels
2. issue 120 orders for xud-bob: xucli sell 0.01 ETH/BTC 0.03 for a total of 1.2 ETH
3. issue 1 buy order from xud-alice: xucli buy 1.2 ETH/BTC 0.03 for a total of 1.2 ETH
4. ERROR: xud-bob does not shutdown gracefully