This is how Raygun manages to peek into window.onerror and dispatch the error to its servers

unobfuscatedRaygunScript.js

!function (
 window, 
 document, 
 scriptString, 
 minifiedRaygunScript, 
 rg4jsString, 
 domScriptElement, 
 firstStriptTag, 
 windowOnError
) {
    window.RaygunObject = rg4jsString,
    window[rg4jsString] = window[rg4jsString] || function () {
            (window[rg4jsString].o = window[rg4jsString].o || []).push(arguments);
        },
    domScriptElement = document.createElement(scriptString),
    firstStriptTag = document.getElementsByTagName(scriptString)[0],
    domScriptElement.async = 1,
    domScriptElement.src = minifiedRaygunScript,
    firstStriptTag.parentNode.insertBefore(domScriptElement, firstStriptTag),
    // from MDN: window.onerror = function(message, sourceScript, lineno, colno, errorObj)
    windowOnError = window.onerror,
    window.onerror = function (msg, sourceScript, lineNumber, columnNumber, errorObj) {
        windowOnError && windowOnError(msg, sourceScript, lineNumber, columnNumber, errorObj),
        errorObj || (errorObj = new Error(msg)),
        window[rg4jsString].q = window[rg4jsString].q || [],
        window[rg4jsString].q.push({
            e: errorObj
        });
    };
}(window, document, 'script', '//cdn.raygun.io/raygun4js/raygun.min.js', 'rg4js');