Backend\Modules\Extensions\Actions\InstallModule::validateInstall PHP Method

    private function validateInstall()
    {
        // already installed
        if (BackendModel::isModuleInstalled($this->currentModule)) {
            $this->redirect(BackendModel::createURLForAction('Modules') . '&error=already-installed&var=' . $this->currentModule);
        }
        // no installer class present
        if (!is_file(BACKEND_MODULES_PATH . '/' . $this->currentModule . '/Installer/Installer.php')) {
            $this->redirect(BackendModel::createURLForAction('Modules') . '&error=no-installer-file&var=' . $this->currentModule);
        }
    }