Qt GUI does not show GUI error and does not quit in case RPC connection is lost after wallet loaded

[kristapsk]

Only error in console is printed, no feedback at all for GUI-only user.

Steps to reproduce:

1. Start Bitcoin node;
2. Start JoinMarketQt;
3. Load JoinMarket wallet;
4. Shutdown Bitcoin node.

I believe problem is return None instead of sys.exit here:

joinmarket-clientserver/jmclient/jmclient/blockchaininterface.py

Lines 214 to 232 in 9a372fd

	try:
	res = self.jsonRpc.call(method, args)
	except JsonRpcConnectionError as e:
	# note that we only raise this in case the connection error is
	# a refusal, or is unrecognized/unknown by our code. So this does
	# NOT happen in a reset or broken pipe scenario.
	# It is safest to simply shut down.
	# Why not sys.exit? sys.exit calls do *not* work inside delayedCalls
	# or deferreds in twisted, since a bare exception catch prevents
	# an actual system exit (i.e. SystemExit is caught as a
	# BareException type).
	log.error("Failure of RPC connection to Bitcoin Core. "
	"Application cannot continue, shutting down.")
	stop_reactor()
	return None
	# note that JsonRpcError is not caught here; for some calls, we
	# have specific behaviour requirements depending on these errors,
	# so this is handled elsewhere in BitcoinCoreInterface.
	return res

I'm not so familiar with Reactor, but could we raise some exception instead? @AdamISZ ?