Q:

An Alternative to 

If a script resource could not be found by <script> tag
<script src="link.to.js"> <!-- 404 or 500>

Do I have an alternative to work around it naturally in html?
edit: the source it's in an external server, like 
<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.6.9/angular.min.js"></script>

A:

There is a way to fallback to another resource if the url fails to load if you are using Service Workers, but that requires https. with service worker you have also the possibility to cache resources and make it available to work offline. (which is a good scenario for things like angular)
self.addEventListener('fetch', function(event) {
  event.respondWith(
    fetch(event.request).then(function(response) {
      if (response.ok) {
        // If we got back a non-error HTTP response, return it to the page.
        return response
      } else {
        // An HTTP error response code (40x, 50x) won't cause the fetch() promise to reject.
        fetch('http://example.com/404')
      }
    })
  )
})

Another easier way would be to use something that i have seen used with jQuery a lot where if a cdn fails you can fallback to a local path
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.2.6/jquery.min.js"></script>
<script>
   window.jQuery || document.write('<script src="/path/to/your/jquery"><\/script>');
</script>

