Updating `DMCSimulationResults.flat_result_iter`.

The parsing choked if `include_tau_exclusion_info` was `True` and there
were GLM-regression failures (i.e. NaN values). Adding logic to use the
unadjusted model index in its place and report a detailed warning. This
should be mostly limited to cases where there is only one model in the
analysis, and these can be avoided by specifying the combined prior
index to `write_result_summaries` method.