Fix ``romt crate --keep-going`` to correctly handle ``403 Forbidden``

• Fix romt crate --keep-going to correctly handle 403 Forbidden and
  other HTTP status failures (thanks to Anthony Gray,
  https://github.com/f34rt3hbunn3h).

  When porting from requests to httpx, the exception handling in the
  Downloader class was incorrectly switched from the requests library's
  RequestException base class to the httpx library's RequestError
  class; the former is the base class for all of the exceptions in requests,
  whereas the latter doesn't cover all exceptions in httpx. This fix
  switches the exception handler to properly use httpx.HTTPError to catch
  all httpx library exceptions.

  References:

  • https://docs.python-requests.org/en/master/_modules/requests/exceptions/
  • https://www.python-httpx.org/exceptions/