github spout: use f3 internals and log errors in db

- Using f3 internals \Web::instance()->request() transparently uses curl
  if available, which may help workaround file_get_contents() getting stuck
  sometimes. This may be https://bugs.php.net/bug.php?id=51330 .
- Raising an exception instead of returning false makes the error logged
  in the db which is nice.