`DepoFluxWriter` exception could be more clear.

[brettviren]

As part of the "collusion" (see https://github.com/LArSoft/larwirecell/blob/develop/larwirecell/docs/depofluxwriter.org#track-ids ) the same art::Event "label" must be used for the vector of SimEnergyDeposit by both SimDepoSetSource and DepoFluxWriter. If the user gets this wrong by naming two different but existing labels then DepoFluxWriter will throw an exception about an out-of-bounds access which is somewhat cryptic.

Better would be to have DepoFluxWriter explicitly check the size of the SED vector against the vector of depos from WCT and raise an exception with a more explicit message.