Bring swaps in critical states to user's attention

[kilrau]

Enhancement of #1081:

When xud crashes between SwapCreated and SwapCompleted: there are certain scenarios where xud can't do much as of now (e.g. on-chain claiming) and the user needs to act manually.

This issue is about defining what critical swap states we have and how we bring them to the user's attention (and what hints we give on what to do) Only mediocre ideas so far: getinfo, SubscribeSwaps event.

[kilrau]

Let's discuss and decide for something on the call

[kilrau]

covered by #1079