textbox: Throw a lua error on invalid markup

Idea by JD, inspired from a patch by farhaven.

Signed-off-by: Uli Schlachter <psychon@znc.in>

widgets/textbox.c

@@ -414,6 +414,9 @@ luaA_textbox_newindex(lua_State *L, awesome_token_t token)
                 }
 
                 d->extents = draw_text_extents(&d->data);
+
+                if(!success)
+                    luaL_error(L, "Invalid markup in '%s'", buf);
             }
             else
                 p_clear(&d->extents, 1);