unknown DB column error received

State: had deactivated/deleted WP2Static core a few times after activating the crawl addon. Didn't do an export initially to confirm if this error is present then, too.

To investigate

```
2020/11/03 11:39:13 [error] 79#79: *115 FastCGI sent in stderr: "PHP message: WordPress database error Unknown column 'crawled_time' in 'where clause' for query SELECT id, url FROM wp_wp2static_urls
WHERE crawled_time IS NULL OR crawled_time <= '2020-11-03 11:39:13'
LIMIT 20 made by do_action('wp_ajax_wp2static_run'), WP_Hook->do_action, WP_Hook->apply_filters, WP2Static\Controller::wp2staticRun, WP2Static\Controller::wp2staticHeadless, WP2Static\Controller::wp2staticCrawl, do_action('wp2static_crawl'), WP_Hook->do_action, WP_Hook->apply_filters, WP2StaticAdvancedCrawling\Crawler::wp2staticCrawl, WP2StaticAdvancedCrawling\Crawler->crawlSite, WP2StaticAdvancedCrawling\CrawlQueue::getChunkPHP message: PHP Fatal error: Uncaught UnexpectedValueException: RecursiveDirectoryIterator::__construct(/usr/html/wp-content/uploads/wp2static-processed-site): failed to open dir: No such file or directory in /usr/html/wp-content/plugins/wp2static-addon-zip/src/ZipArchiver.php:30
Stack trace:
#0 /usr/html/wp-content/plugins/wp2static-addon-zip/src/ZipArchiver.php(30): RecursiveDirectoryIterator->__construct('/usr/html/wp-co...', 4096)
#1 /usr/html/wp-content/plugins/wp2static-addon-zip/src/Controller.php(87): WP2StaticZip\ZipArchiver->generateArchive('/usr/html/wp-co...')
#2 /usr/html/wp-includes/class-wp-hook.php(287): WP2StaticZip\Controller->generateZip('/usr/html/wp-co...', 'wp2static-addon...')
#3 /usr/html/wp-includes/class-wp-hook.php(311): WP_Hook->apply_filters(NULL, Array)
#4 /usr/html/wp-includes/plugin.php(478): WP_Hook->do_action(Array)
#5 /usr/html/wp-content/plugins/wp2static/src/Controller.php(635): do_action('wp2static_deplo...', '/usr/html/wp-co...', 'wp2static-addon...')
#6 /usr/html/" while reading response header from upstream, client: 172.17.0.1, server: , request: "POST /wp-admin/admin-ajax.php HTTP/1.1", upstream: "fastcgi://unix:/var/run/php7-fpm.sock:", host: "localhost:4588", referrer: "http://localhost:4588/wp-admin/admin.php?page=wp2static"
```