Make error message more useful

Change-Id: I3c6efcbd2a2759385912aa352d0c250963e5f004
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/148464
Tested-by: Jenkins
Reviewed-by: Tor Lillqvist <tml@collabora.com>

desktop/source/lib/init.cxx

@@ -3213,7 +3213,7 @@ static int doc_saveAs(LibreOfficeKitDocument* pThis, const char* sUrl, const cha
             }
             else
             {
-                SetLastExceptionMsg("input filename without a suffix");
+                SetLastExceptionMsg("input URL '" + aURL + "' lacks a suffix");
                 return false;
             }
         }