improve error reporting

        dut_output_file = f"{dut_base_path}/param_file/dec/{output_file}"

        ref_output_file = f"{reference_path}/param_file/dec/{output_file}"

        fs = int(sampling_rate) * 1000

        cmp_result, reason = cmp_pcm(dut_output_file, ref_output_file, output_config, fs)

        assert cmp_result == 0, reason

        output_differs, reason = cmp_pcm(dut_output_file, ref_output_file, output_config, fs)

        metadata_differs = False

        for md_file in md_out_files:

            print(md_file)

            dut_metadata_file = f"{dut_base_path}/param_file/dec/{md_file}"

            ref_metadata_file = f"{reference_path}/param_file/dec/{md_file}"

            msg = "Metadata output files differ between reference and dut."

            assert filecmp.cmp(dut_metadata_file, ref_metadata_file), msg

            metadata_differs = not filecmp.cmp(dut_metadata_file, ref_metadata_file)

        if output_differs or metadata_differs:

            msg = "Difference between ref and dut in "

            if output_differs and metadata_differs:

                msg += "ouput and metadata"

            elif output_differs:

                msg += "output only"

            elif metadata_differs:

                msg += "metadata only"

            assert False, msg

        # remove DUT output files when test result is OK (to save disk space)

        if not keep_files: