This commit adds error returns to all of the Db interface methods except
for FetchTxByShaList and FetchUnSpentTxByShaList since they expose the
errors on each individual transaction.
It also updates all tests and code for both the leveldb and memdb drivers
for the changes.
Closes#5.
ok @drahn
improved error handling
Fix testing, no point in running tests multiple times
Fix error when no blocks in database, reopen would misreport NewstSha() return
