You can add a variable inside http://fail.org/nonexistant.js like var isExecuted = true; and then check if it exists when the script tag is loaded:

var isExecuted = false;
script_tag.onload = script_tag.onreadystatechange = function() {
    if(!this.readyState ||
        this.readyState == "loaded" || this.readyState == "complete") {
        // script successfully loaded
        
        if(isExecuted) // no error
    }
}

To check if the javascript in nonexistant.js returned no error you have to add a variable inside http://fail.org/nonexistant.js like var isExecuted = true; and then check if it exists when the script tag is loaded.

However if you only want to check that the nonexistant.js returned without a 404 (meaning it exists), you can try with a isLoaded variable ...

var isExecuted = false;
var isLoaded = false;
script_tag.onload = script_tag.onreadystatechange = function() {
    if(!this.readyState ||
        this.readyState == "loaded" || this.readyState == "complete") {
        // script successfully loaded
        isLoaded = true;
        
        if(isExecuted) // no error
    }
}

This will cover both cases.

var isError = true;
script_tag.onload = script_tag.onreadystatechange = function() {
    if(!this.readyState ||
        this.readyState == "loaded" || this.readyState == "complete") {
        // script successfully loaded
        isError = false; // no error
    }
}

This works cross-browser.

You can add a variable inside http://fail.org/nonexistant.js like var isExecuted = true; and then check if it exists when the script tag is loaded:

var isExecuted = false;
script_tag.onload = script_tag.onreadystatechange = function() {
    if(!this.readyState ||
        this.readyState == "loaded" || this.readyState == "complete") {
        // script successfully loaded
         
        if(isExecuted) // no error
    }
}