True if an HTTP GET request to Url produces a Response.
Url can be an atom, string or list of codes. If Response is
an unbound variable, it's unified with a value representing
the full HTTP response. See
library(response) in this pack
for predicates about this value.
Response may also be one of the following:
codes(Codes) - returns response body as a list of codes
status_code(Code) - unifies Code with the HTTP status code
html5(Dom) - parses response body as HTML using load_structure/3
json(Dict) - parses response body as JSON
[...] - a list of the above; only one may parse the response body