This changes the verify checkpoints fail handling to use the
GUI reset blockchain facility if in the GUI, or direct the user
to use the -resetblockchaindata option if using the daemon,
rather than instructing the user to manually delete the blockchain
data files.

This will cause the wallet to ShowChainCorruptedMessage() and
either delete the blockchain files (in the GUI case) and end, or
post a log entry with instructions and end.

A missing pprev on any blockindex entry is fatal and the wallet should
not be allowed to continue operation without rebuilding the chain
data.