Add curation remark table and refactor error mgmt.

The curator requires the existence of a curator remark table, which
stores the progress for a given curation policy.  The tests for the
curator create an ad hoc table, but core Prometheus presently lacks
said table, which this commit adds.

Secondarily, the error handling for the LevelDB lifecycle functions
in the metric persistence have been wrapped into an UncertaintyGroup,
which mirrors some of the functions of sync.WaitGroup but adds error
capturing capability to the mix.
Merge pull request #197 from prometheus/feature/storage/curation-table

Add curation remark table and refactor error mgmt.