Show warning if HTTPS request fails

In `Core_Command`, `download` first [tries](https://github.com/wp-cli/wp-cli/blob/master/php/commands/core.php#L71) to access the URL via HTTPS with verification, then falls back to ignoring the certificate. By doing this without a warning, it's basically equivalent to always ignoring verification. (Ditto in `_read`)

As of 1.6, Requests has browser-level SSL verification, so any issues will be with the certificate, not with the library. Hence, any errors here should either kill the request immediately or at least show a warning to the user.