Add support for pagination of errors

Previously errors could only be listed as all errors since a given time.
Now this is augmented with the ability to paginate the list, and show
only a certain range of errors.

This change is as-yet untested.
Merge branch 'master' of github.com:BitShares/bitshares_toolkit
Add clear_errors API call

Create a new API call to remove some set of errors from the log.