ensuringArgumentIsUrl
Ensures the argument at index is an atom parsing (via Url.of) into a valid source/sink Url, as required by open/3,4 and consult/1.
Throws
if it is unbound.
(source_sink) if it is bound but not an atom, or is an atom that does not parse into a valid Url (including as a bare filesystem path, see Url.Companion.of).