luci-base: expose sysauth rendering errors

In case a theme shipped sysauth.htm failed to render/execute, expose the
exception error details in the ui theme fallback indicator.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

modules/luci-base/ucode/dispatcher.uc

@@ -921,6 +921,7 @@ dispatch = function(_http, path) {
 						runtime.render(`themes/${basename(runtime.env.media)}/sysauth`, scope);
 					}
 					catch (e) {
+						runtime.env.media_error = `${e}`;
 						runtime.render('sysauth', scope);
 					}