download: Continue handshake upon TLS warning alerts.

This allows us to download from site such as
<https://fusionforge.int-evry.fr> where the server does not recognize
the server name passed via the 'server_name' extension.

* guix/build/download.scm (tls-wrap): Catch 'gnutls-error' around
'handshake'.  Upon ERROR/WARNING-ALERT-RECEIVED, print a message and
call 'handshake'.