Merge branch 'jk/http-error-messages'

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