rollback upon error request

[samseaver]

I encountered a rare case when importing a file to have the import process crap out on me. This left a lot of reactions in the REACTION_MODEL table for that model, and thus I could query the incomplete model. I probably shouldn't be able to do that, there'd be all kinds of weird things happen, so I'm writing to make sure that the biochemistry for a model is "rolled-back" if the code fails.