Document BAD_INPUT_DATA error in key material exporter

Signed-off-by: Max Fillinger <maximilian.fillinger@foxcrypto.com>
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 32fbe13..0c3fc84 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -10147,6 +10147,7 @@
                                        const int use_context)
 {
     if (!mbedtls_ssl_is_handshake_over(ssl)) {
+        /* TODO: Change this to a more appropriate error code when one is available. */
         return MBEDTLS_ERR_SSL_BAD_INPUT_DATA;
     }