nalutils: Improve slightly the error trace

nalutils: Improve slightly the error trace

Until now, bound check would simply trace the values and the range. This
enhances the trace by also tracing the name of the variable that was to be set
or read. This is not magically perfect in all cases, but greatly speed the
debugging work. Here's an example before and after this change:

Before: gst_h264_parser_parse_slice_hdr: value not in allowed range. value: 819183, range -87-77
After:  gst_h264_parser_parse_slice_hdr: value for 'slice->slice_qp_delta' not in allowed range. value: 819183, range -87-77
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1356>