Fix crash on startup when the config file can't be written

src/frame_main.cpp

@@ -283,7 +283,12 @@ FrameMain::FrameMain()
 #ifdef WITH_UPDATE_CHECKER
 		int result = wxMessageBox(_("Do you want Aegisub to check for updates whenever it starts? You can still do it manually via the Help menu."),_("Check for updates?"), wxYES_NO | wxCENTER);
 		OPT_SET("App/Auto/Check For Updates")->SetBool(result == wxYES);
+		try {
 			config::opt->Flush();
+		}
+		catch (agi::fs::FileSystemError const& e) {
+			wxMessageBox(to_wx(e.GetMessage()), "Error saving config file", wxOK | wxICON_ERROR | wxCENTER);
+		}
 #endif
 	}