Mac: Fix error message for haptic subsystem.

We were calling SDL_Error instead of SDL_SetError when the haptic subsystem
wasn't initialized.

src/haptic/darwin/SDL_syshaptic.c

@@ -156,7 +156,7 @@ SDL_SYS_HapticInit(void)
     io_service_t device;
 
     if (numhaptics != -1) {
-        return SDL_Error("Haptic subsystem already initialized!");
+        return SDL_SetError("Haptic subsystem already initialized!");
     }
     numhaptics = 0;