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 is returned.
the alert code to use for a particular error code.