Improve Exception handling

[ayyaruq]

Exceptions are currently spread between nari.utils and nari.io.reader.actlogutils, which is fine in theory except we're using ACT stuff out of the ACT specific code (last I checked). We probably need to structure these better and think thru which exceptions we need or are likely to need.

[ayyaruq]

Update on this, now that the ACT stuff has been split out, the exceptions have been cleaned up a little. Prob need a pass over the names to make sure they're sensible, and consider what other exceptions we might want, but the structural separation is fine as of #100.