bitcoind: gracefully handle incorrect credentials

[Sjors]

I was treated to a crash and cryptic stack trace:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Json(unexpected eof)', src/libcore/result.rs:1009:5
stack backtrace:

The bitcoind debug log explained what's going on:

2019-01-28T11:44:58Z ThreadRPCServer incorrect password attempt from 127.0.0.1:54517

When catching this error, it's helpful to add an additional debug line saying Using rpcuser <RPCUSER> and rpcpassword=**** from <DATADIR>/rbf.conf, so the user knows they didn't put the configuration in the wrong place.

It may also be helpful to add a testbitcoinrpc command, which could also check if a wallet and transaction index are present (by calling getrawtransaction on something really old).