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.