Fetches a URL with an HTTP GET and reports the result through a callback rather
than a promise, since that is what the fetch layer expects everywhere else.
Parameters
url: string
The URL to fetch.
callback: (err:string|null,body?:string)=>void
Called once with an error, or with null and the response body on success.
Fetches a URL with an HTTP GET and reports the result through a callback rather than a promise, since that is what the fetch layer expects everywhere else.