Update Agent Status on failed agent-status tasks
Update Agent Status on failed agent-status tasks

Now, if an agent-status task fails, the associated (by-address) agent
record will be updated with the status 'error', to indicate that
something is broken in the checking process.

Fixes #594

starkandwayne/shield,starkandwayne/shield,starkandwayne/shield,starkandwayne/shield,starkandwayne/shield