The best way for this is to listen for the error event of the first script tag. You can do this for example with the onerror property:

<script src="https://mipage.com/pathtoscript/owlscript.js" onerror="loadFallbackScript()"></script>

and then loading the fallback script from code:

function createScriptElement(src) {
    const script = document.createElement('script');
    script.src = src;

    return script;
}

function loadFallbackScript() {
    const fallbackScriptElement = createScriptElement('fallback.js');
    document.head.append(fallbackScriptElement);
}

But since this requires loadFallbackScript to be a global function a cleaner way would be to create the first script tag dynamically as well and attach the event listener from code:

// execute this instead of the <script> tag in the html
const externalScriptElement = createScriptElement('external.js');
externalScriptElement.addEventListener('error', loadFallbackScript);
document.head.append(externalScriptElement);