When a command exits with status 1 it doesn't stop gnome-settings-daemon from processing the input device.

Issue
=====
In run_custom_command, when spawned process exits with code 1, the exit_status variable is set to 256

Test:
echo -e '#include <stdlib.h>\nvoid main(){exit(1);}' > test_exit_code.c
gcc test_exit_code.c -o test_exit_code
dconf write /org/gnome/settings-daemon/peripherals/input-devices/hotplug-command "'`pwd`/text_exit_code'"

// Now run g-s-d inside gdb:
gdb --args /usr/libexec/gnome-settings-daemon
# Set breakpoint (the line is as found in Gnome 3.8.4
(gdb) b gsd-input-helper.c:531
(gdb) r
# Plug in device and wait for gdb to break 
(gdb) p exit_status
$1 = 256 # Should be 1.


Origin
=======
It appears that in POSIX the exit status has two bytes. The lowest byte is the signal that stopped the program if any. The highest byte is the exit status.

http://docs.python.org/2/library/os.html#os.wait
http://en.wikipedia.org/wiki/Exit_status#POSIX

It is the same for the input-device-example.sh in the g-s-d source code.
It also behaves the same for Python scripts:
echo -e '#!/usr/bin/python\nimport sys\nsys.exit(1)' > test_exit_code
gcc test_exit_code.c -o test_exit_code


Possible solutions
==================
Either:
* Change the semantics of run_custom_command to return TRUE if (exit_code!=0)
* Get actual error code with https://developer.gnome.org/glib/stable/glib-Spawning-Processes.html#g-spawn-check-exit-status
* Something else?

Daniel