I'm generally against changing behavior that works today and has been in production for a year. So, I'm a bit torn.

In any case, AAE is designed to be lossy. Dropping updates/etc is no big deal. However, if I can't put to a LevelDB instance, something's seriously wrong. I imagine future attempts to iterate over the LevelDB instance (eg. for an exchange) would fail as well. Hard to imagine a scenario where `eleveldb:put` fails, but other operations against the tree work just fine. So, this is seems to be an error we might as well fail against.

If it somehow ends up breaks things in production, it's a 1 line change in a 2.0.1, right? ;)