Print query errors to stderr instead of stdout

[dominiklohmann]

Bug report

I was trying to implement #104 properly with the new attributes and came across an issue.

If yabai -m query --spaces --display $sel fails, it prints "could not locate the selected display." to stdout, which messes with my script.

This does affect quite a few error messages all throughout yabai. They all need to print to stderr instead.

[koekeishiya]

yabai itself always outputs warnings / errors to stderr. Messages returned to the client was erroneously using stdout for error reporting. Fixed on master.