uui: show what is the unhandled result before asserting

online.git unit-bad-doc-load fails here, at least show what is the
unhandled return code (-1) before failing.

The actual problem is not yet solved.

Change-Id: I530058590f176f5c7eee3b6cd9d9a270110613ed
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/163436
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins

uui/source/iahndl.cxx

@@ -889,6 +889,7 @@ executeMessageBox(
             aResult = DialogMask::ButtonsNo;
             break;
         default:
+            SAL_WARN("uui", "executeMessageBox: nMessResult is " << nMessResult);
             assert(false);
     }