stop gracefully instead of trying to resume ops
stop gracefully instead of trying to resume ops

Refs #2072

We most probably shouldn't be running any further when there is some
unexpected panic. Some unknown error happened, and so we don't know if
that will result in the validator signing an invalid thing. It might be
worthwhile to explore a mechanism for manual resuming via some console
or secure RPC system, but for now, halting the chain upon unexpected
consensus bugs sounds like the better option.

tendermint/tendermint,tendermint/tendermint,tendermint/tendermint,tendermint/tendermint