Improve error message: Workflow executionsRow not found

[lorensr]

Is your feature request related to a problem? Please describe.

In this scenario, the error from the Worker that had the correct code was:

Task not found when completing: status: NotFound, message: "Workflow executionsRow not found."

Describe the solution you'd like

Say what's going wrong in the Worker at a higher level and suggest possible reasons.

Migrated from #359

[bergundy]

This is a problem with the server AFAICT, I think you get a different error message if you use postgres instead of cassandra.