Provide a better `bitcoin-cli` error message when `bitcoind` is not running.

Before this patch:

```
$ killall -9 bitcoind
$ bitcoin-cli -testnet echo 'hello world'
error: Could not locate RPC credentials. No authentication cookie could be found, and RPC password is not set.  See -rpcpassword and -stdinrpcpass.  Configuration file: (/root/.bitcoin/bitcoin.conf)
```

After this patch:

```
$ killall -9 bitcoind
$ bitcoin-cli -testnet echo 'hello world'
error: Could not connect to the server 127.0.0.1:18332

Make sure the bitcoind server is running and that you are connecting to the correct RPC port.
```

test_runner.py

./bitcoind
kill <bitcoind pid>
./bitcoin-cli <any rpc command>