sd-bus: Exit event loop with error code instead of EXIT_FAILURE

sd-bus: Exit event loop with error code instead of EXIT_FAILURE

Instead of failing the event loop with a generic EXIT_FAILURE
error code when exit-on-disconnect is used, let's propagate the
error code instead of swallowing it.

Whereas previously sd_event_loop() would always fail with exit code
'1' when exit-on-disconnect is used with an sd-bus instance registered
with the event loop that encounters a failure, now we'll correctly
propagate the error to sd_event_loop() that caused sd-bus to fail and
exit the event loop. Additionally, the error is now also properly
propagated to outstanding reply callbacks for async dbus calls started
with sd_bus_call_async() and friends, whereas before we always used
ETIMEDOUT for these calls which is extremely confusing for users.

Why is this confusing? We always start sd-bus instances asynchronously,
in other words, sd_bus_start() will not actually wait until the bus instance
is connected, but it'll happen in the background, either driven by the first
sd_bus_call() when there is no event loop or by sd-event when there is an
event loop attached to the sd-bus instance. Assuming an event loop is attached,
when we fail to connect to the bus, the sd-bus instance will close down and the
first async method call we queued will fail with ETIMEDOUT. Nowhere in this process
do we inform the user that we failed to connect to the bus because of e.g. a permission
error, except for a debug log message.

By propagating the error to sd_event_exit() if exit-on-disconnect is enabled
and always propagating it to outstanding reply callbacks, debugging failures
becomes much easier as users will now get the actual error code causing the
bus instance to close down instead of ETIMEDOUT and 1 respectively.