Errors processing device list changes are swallowed

If there is a problem processing the `device_lists` part of a `/sync` response, the error is logged, but otherwise ignored. This is problematic because it means that, at some point in the future, we will cause unable-to-decrypt events.

It would be better to keep retrying the `/sync`, or to show an error box, than just to just carry on regardless, storing up problems for the future.

(We recently got UTD reports which turned out to be due to a [now-fixed bug](https://github.com/rustwasm/wasm-bindgen/issues/4352) in wasm-bindgen. The problem here isn't so much the bug in wasm-bindgen, as that other, similar bugs could cause UTDs.)