Add detail to error if webchat url doesn't exist

This will allow us to pinpoint which GOV.UK pages are trying to open a
webchat url, without having any urls defined in [webchat.yaml]

[webchat.yaml]: https://github.com/alphagov/government-frontend/blob/5974c4ec2c7e7a4eedcfda8f48370cce79c8efa0/lib/webchat.yaml

app/assets/javascripts/webchat/library.js

@@ -21,7 +21,7 @@
 
     function init () {
       if (!availabilityUrl || !openUrl) {
-        throw Error.new('urls for webchat not defined')
+        throw Error.new('urls for webchat not defined', window.location.href)
       }
 
       if (openButton) {