Store URL information in JavaScriptError class

[borislubimov]

Thank you for the great API. Not long ago I've started use it and found several js-errors with it. When I wrote bug-reports about found errors I realized that it's not always obvious to find the page on which the js-error occured if we read errors after visit more than one page. Therefore it would be great if JavaScriptError class collects not only errorMessage, sourceName and lineNumber but also URL of the page with an error too.