Implement fatal error message box

Used when lite-xl fails at startup

data/core/init.lua

@@ -406,7 +406,7 @@ function core.init()
     end
     project_dir_abs = system.absolute_path(".")
     if not core.set_project_dir(project_dir_abs) then
-      print("internal error: cannot set project directory to cwd")
+      system.show_fatal_error("Lite XL internal error", "cannot set project directory to cwd")
       os.exit(1)
     end
   end

src/api/system.c

@@ -258,6 +258,30 @@ static int f_show_confirm_dialog(lua_State *L) {
 }
 
 
+static int f_show_fatal_error(lua_State *L) {
+  const char *title = luaL_checkstring(L, 1);
+  const char *msg = luaL_checkstring(L, 2);
+
+#ifdef _WIN32
+  MessageBox(0, msg, title, MB_OK | MB_ICONERROR);
+
+#else
+  SDL_MessageBoxButtonData buttons[] = {
+    { SDL_MESSAGEBOX_BUTTON_RETURNKEY_DEFAULT, 0, "Ok" },
+  };
+  SDL_MessageBoxData data = {
+    .title = title,
+    .message = msg,
+    .numbuttons = 1,
+    .buttons = buttons,
+  };
+  int buttonid;
+  SDL_ShowMessageBox(&data, &buttonid);
+#endif
+  return 0;
+}
+
+
 static int f_chdir(lua_State *L) {
   const char *path = luaL_checkstring(L, 1);
   int err = chdir(path);
@@ -441,6 +465,7 @@ static const luaL_Reg lib[] = {
   { "set_window_size",     f_set_window_size     },
   { "window_has_focus",    f_window_has_focus    },
   { "show_confirm_dialog", f_show_confirm_dialog },
+  { "show_fatal_error",    f_show_fatal_error    },
   { "chdir",               f_chdir               },
   { "mkdir",               f_mkdir               },
   { "list_dir",            f_list_dir            },

src/main.c

@@ -134,8 +134,13 @@ init_lua:
     "  core.init()\n"
     "  core.run()\n"
     "end, function(err)\n"
-    "  print('Error: ' .. tostring(err))\n"
-    "  print(debug.traceback(nil, 2))\n"
+    "  system.show_fatal_error('Lite XL start error', '"
+    "Fatal error: cannot locate the data directory.\\n"
+    "Please verify that the data folder is available.')\n"
+    "  local fp = io.open((exedir and exedir .. '/' or '') .. 'error.txt', 'wb')\n"
+    "  fp:write('Error: ' .. tostring(err) .. '\\n')\n"
+    "  fp:write(debug.traceback(nil, 2) .. '\\n')\n"
+    "  fp:close()\n"
     "  if core and core.on_error then\n"
     "    pcall(core.on_error, err)\n"
     "  end\n"