Last Sync - Improvements

[bh2smith]

@MartinquaXD raised some good (non-blocking) points of improvement for the already merged code in #12. Here I will try to summarize;

1. No need for Optional Block Number. This is only used for testing purposes and the tests could be adapted instead of the return type.
2. Throw error on invalid table name. I think the problem with this may be that our test tables write to a different directory (intentionally) so not to interfere with real data. I think we may want to keep this.... not sure.
3. Look at when KeyError occurs
4. Use Contract Deployment as default genesis block

I think items 1 and 4 are the most straight forward to change first, still not sure yet about the other two.