404 for an id that is not a web user

http://manage.dimagi.com/default.asp?145627#812259

Now 404s if there it you give a wrong id for a WebUser (like an id for a group or something)
i don't feel super strongly but it feels kind of funny to be throwing request-specific exceptions in such a general function. it may be that other callers want to handle this error entirely differently (though i don't have a concrete use case for that). i might advocate for using `get_document_or_404` in the view in favor of this change.