Shield should quietly resume after stream-sync errors

[JSKitty]

Currently, if the stream errors during Shield Sync, the wallet either seems to restart the sync entirely (due to hitting 'Bad root' logic) or causes the wallet to enter a bad sync state, where it soft-locks.

This only seems to be an issue on extremely weak or slow connections, but it is still a problem that could potentially lock out some users or make their usage of MPW very inconsistent.

Ideally, this should be handled by resuming from the last Shield block synced, quietly, making it unnoticable for users.