ion will send an alert to the peer in order to inform him
     of something important (eg.  his Certificate could not be
     verified).  If the alert level is Fatal then the peer is expected
     to close the connection, otherwise he may ignore the alert and
     continue.

     The error code of the underlying record send function will be
     returned, so you may also receive ‘GNUTLS_E_INTERRUPTED’ or
     ‘GNUTLS_E_AGAIN’ as well.

     *Returns:* On success, ‘GNUTLS_E_SUCCESS’ (0) is returned,
     otherwise an error code is returned.

 -- Function: int gnutls_error_to_alert (int ERR, int * LEVEL)
     ERR: is a negative integer

     LEVEL: the alert level will be stored there

     Get an alert depending on the error code returned by a gnutls
     function.  All alerts sent by this function should be considered
     fatal.  The only exception is when ‘err’ is ‘GNUTLS_E_REHANDSHAKE’
     , where a warning alert should be sent to the peer indicating that
     no renegotiation will be performed.

     If there is no mapping to a valid alert the alert to indicate
     internal error (‘GNUTLS_A_INTERNAL_ERROR’ ) is returned.

     *Returns:* the alert code to use for a particular error code.