## Issue Addressed
Closes#2028
Replaces #2059
## Proposed Changes
If writing to the database fails while importing a block, revert fork choice to the last version stored on disk. This prevents fork choice from being ahead of the blocks on disk. Having fork choice ahead is particularly bad if it is later successfully written to disk, because it renders the database corrupt (see #2028).
## Additional Info
* This mitigation might fail if the head+fork choice haven't been persisted yet, which can only happen at first startup (see #2067)
* This relies on it being OK for the head tracker to be ahead of fork choice. I figure this is tolerable because blocks only get added to the head tracker after successfully being written on disk _and_ to fork choice, so even if fork choice reverts a little bit, when the pruning algorithm runs, those blocks will still be on disk and OK to prune. The pruning algorithm also doesn't rely on heads being unique, technically it's OK for multiple blocks from the same linear chain segment to be present in the head tracker. This begs the question of #1785 (i.e. things would be simpler with the head tracker out of the way). Alternatively, this PR could just revert the head tracker as well (I'll look into this tomorrow).
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