Fixes and improvments

Fixes and improvments

- Throw a TextException on errors in Ssl::SSL_add_untrusted_cert
- Move most of the code from ConnStateData::start() into the new virtual method
  ConnStateData::prepUserConnection. Call this method instead calling
  grandparents ::start() method from Downloader::start().
  Also implement an empty Downloader::prepUserConnection() method
- Fix Downloader::isOpen() to use doneAll() to check if its jobs is finished
  and its job assumed as closed.
- Fix Downloader::start to handle the case a wrong HTTP request passed from
  the user (eg malformed URL). In this case calaback to the user with an
  Http::scInternalServerError.
- Remove tbe Downloader::callException() method, existed only to print a
  debug message. Add a debug message in AsyncJob::callException method instead.
- Replaces NULL with nullptr
- Fixes debug messages and comments