Calling t.Fatal inside db.View makes the test hang on failures.  Return
the error and call t.Fatal outside of db.View avoids this.

Pruning stale blocks will make the block validation fail for the block
that the prune was triggered on as BlockHeightByHash will not return the
height for blocks that are not in the main chain.

We add a test case to ensure that the test fails in the above case.

calculations

Since BlockHeightByHash only returns the heights for blocks that are in
the main chain, when a block that is stale gets pruned, this will cause
an error in the block height lookup and cause an error in block
processing.

Look up the node directly from the index and if the node isn't found,
just skip that node. For utxoCache.lastFlushHash, if that isn't found,
just force a flush.