public FromUrl setCallback(final Callback<Void, Exception> callback) {
            this.callback = callback;
            return this;
        }
Specify a callback to be invoked when the script is loaded or loading
encounters an error.
<p>
<b>Warning:</b> This class <b>does not</b> control whether or not a
URL has already been injected into the document. The client of this
class has the responsibility of keeping score of the injected
JavaScript files.
<p>
<b>Known bugs:</b> This class uses the script tag's <code>onerror()
</code> callback to attempt to invoke onFailure() if the browser
detects a load failure. This is not reliable on all browsers (Doesn't
work on IE or Safari 3 or less).
<p>
On Safari version 3 and prior, the onSuccess() callback may be
invoked even when the load of a page fails.
<p>
To support failure notification on IE and older browsers, you should
check some side effect of the script (such as a defined function) to
see if loading the script worked and include timeout logic.

@param callback callback that gets invoked asynchronously.
stroom-core-client/src/main/java/stroom/dashboard/client/vis/MyScriptInjector.java