Change PlaybackSession createFromOld to use upsert instead of create

[mikiher]

This fixes #2662

While I'm not in the position to reproduce this (I don't own an iPhone), I'm pretty sure why the crash happens.

It is very likely due to duplicate /session/local requests (retries?) sent in rapid succession from the client and handled asynchronously on the server, causing a race condition.

While Request A is awaiting

let session = await Database.getPlaybackSession(sessionJson.id)

Request B is also making the same call.
both receive a null response, so each of them calls create() on the PlaybackSession model, and one of them succeeds while the other fails with UniqueConstraintError because it is trying to create a row with the same primary key.

The fix is simple - since the requests are likely identical, it is safe to use upsert instead of create, which will succeed for both A and B (this of course doesn't prevent the race condition - doing that would involve using transactions, which I think is an overkill in this case)

[mikiher]

Also added a small unrelated change to logging of unhandled rejections, so that the full error object is printed, not just the error message.

[advplyr]

Yeah the local sync endpoint is susceptible to a race condition. The regular sync is not since creating the session and syncing the session are separate requests.

Thanks!