[conformal/btcdb] 16dc2c: Add errors to all interface methods.

[Dave Collins]

  Branch: refs/heads/master
  Home:   https://github.com/conformal/btcdb
  Commit: 16dc2cf2d0231af4015546c6ef91d7ea3710a6f8
      
https://github.com/conformal/btcdb/commit/16dc2cf2d0231af4015546c6ef91d7ea3710a6f8
  Author: Dave Collins <da...@conformal.com>
  Date:   2014-07-07 (Mon, 07 Jul 2014)

  Changed paths:
    M db.go
    M interface_test.go
    M ldb/block.go
    M ldb/boundary_test.go
    M ldb/dup_test.go
    M ldb/insertremove_test.go
    M ldb/leveldb.go
    M ldb/operational_test.go
    M ldb/tx.go
    M memdb/memdb.go
    M memdb/memdb_test.go

  Log Message:
  -----------
  Add errors to all interface methods.

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