[Rivet] YODA excptions

[David Grellscheid]

Hi Andy,

> I also took David's email to mean this. Not sure what is meant by the
> "new" YODA stance on this -- we have raised exceptions for a long time.

Sorry for adding to the confusion! I mixed up the "new" NaN write-outs 
with throwing exceptions. Both are good as they are in YODA, but Rivet 
needs to handle them carefully.

> But like Chris, I'm not sure that this is a great idea, at least not
> with the current production version

I agree. That's what I meant by handling properly. There needs to be a 
try - catch block at an appropriate level to isolate analyses from one 
another.

> -- how do we avoid ending up with histograms in an inconsistent state

In my thinking, that analysis as a whole is dead at that point. Any 
inconsistent histos lead to abandoning the analysis. Once all analyses 
are dead, you stop the run, but not before.

Most of the time, the bug is not in the data coming in, but in a badly 
written analysis that doesn't catch edge-cases properly. In that case 
it's quite rude to kill the whole production run rather than just the 
culprit. :-) Remember that we don't run Rivet stand-alone but in the 
generator run directly.

See you,

   David