The RavenJS Client expects the sentry urls to look like:

http://user:pass@sentry.com/12345

where 12345 is the actual project ID.  It performs a regular expression validation on the url as a parameter and extracts that ID on its end and then makes a request to us with a url of that structure.  Our endpoint is basically pretending it's a sentry endpoint so it has to accept a URL of that structure.