Improve error reporting from the http transfer clients.
* jk/http-error-messages:
http: drop http_error function
remote-curl: die directly with http error messages
http: re-word http error message
http: simplify http_error helper function
remote-curl: consistently report repo url for http errors
remote-curl: always show friendlier 404 message
remote-curl: let servers override http 404 advice
remote-curl: show server content on http errors
http: add HTTP_KEEP_ERROR option