Catch loadblockindex fail

The moneysupply tracking changes changed the record format of the block database.

When upgrading, users will need to -reindex, or they get an ugly error message.

This pull request makes the error message better, at least in clamd. I'm not sure how it will look in clam-qt, but:

    $ clamd
    Error: Error loading block index database : try running with -reindex
    $