Return values from the server callback are interpreted as follows:
+=over 4
+
=item > 0
PSK identity was found and the server callback has provided the PSK
connection will fail with decryption_error before it will be finished
completely.
-=item 0
+=item Z<>0
PSK identity was not found. An "unknown_psk_identity" alert message
will be sent and the connection setup fails.
+=back
+
=cut