Implement error window on OS X

[historic_bruno]

Currently when the game encounters an assertion failure, the OS X crash report window is displayed instead of a message window as on other OSes. This comment in try_gui_display_error() in unix.cpp explains the reason:

// TODO: implement this, in a way that doesn't rely on X11
// and doesn't occasionally cause crazy errors like
// "The process has forked and you cannot use this
// CoreFoundation functionality safely. You MUST exec()."

One possible solution is to use ​NSAlert which is the preferred method of alterting the user with a modal dialog, being careful about thread-safety issues presented by using the AppKit framework.

[historic_bruno]

Example of what I've got so far: ​http://i.imgur.com/c6IjJ.png

[historic_bruno]

On second thought, the OS X error report window is far more informative and useful than anything we could hack up, so it would be nice to keep that if possible (though it does currently interfere with debugging by forcing the game close).