waitForSync reports synced when the only peer has disconnected #86

Part of #80.

$sync.waitForSync('full') only checks connected peers, so if the only peer drops part way through it resolves as "synced" with nothing left to check. Anything that relies on that promise (the cloud's tests, scripts, server-side callers) believes sync completed when, for example, 24 of 100 observations arrived. The mobile Exchange screen derives its stage from the peer count, so users do not see this directly, but it hides interrupted syncs from the code that could react to them.

Fix:

• fix(sync): reject waitForSync when all peers disconnect before sync completes comapeo-core#1352 makes waitForSync reject with a peer-disconnected error when the peer count drops to zero before sync completes. Joining a project then fails cleanly if the inviting device disconnects mid-way, rather than silently keeping a half-synced project.