No lookup_browse/source_url for lookup::* functions

[jdanbrown]

This error might already be what you meant in #20, but I thought I'd flag in case it's not:

> lookup(lookup_browse)
lookup::lookup_browse [closure]
function (x = the$last_lookup)
{
    stopifnot(inherits(x, "lookup"))
    browseURL(tail(source_url(x), n = 1))
}
<environment: namespace:lookup>

> lookup_browse(lookup(lookup_browse))
Error in browseURL(tail(source_url(x), n = 1)) :
  'url' must be a non-empty character string

> source_url(lookup(lookup_browse))
character(0)

(Thanks for the great package!)

[jimhester]

Yeah I think you are right, but it is good to capture the error message as well in an issue, thanks!