Reasoning for this change: to ensure that Qt will show
a message and gracefully exit (or quit attempting to
load a wallet) in all 3 cases: on startup it show an
intelligible message if the RPC connection fails (as
before this PR), if the RPC fails while no wallet is
loaded and thus no wallet service is started, it should
show an intelligible error message when you attempt to
load a wallet and it fails, and finally it should show
an intelligible error message before quitting, if the rpc
connection fails during the period when the wallet is
already loaded.
By switching to an Exception instead of sys.exit, it does
mean that starting a yieldgenerator shows a stack trace,
but it also shows an intelligible error message (in red),
and this is command line, so UI requirements are less strong.
We preserve the "good" behaviour of no stack trace, but
only error message, if the rpc connection is lost during
running.