In cases where a tool fails to run (for example, there are no running workers

that support that tool, or the broker has decided to stop passing messages due

to a full disk), we still want some feedback that Review Bot has tried to start

the process.
This change makes it so the extension creates the StatusUpdate instance, which

is then updated by the worker. When it's first created, the description will be

"starting...", and once the worker actually takes over, that will be updated to

"running...". In a future change, I'm going to make use of the new timeout

feature so that when the task fails to start, it will eventually show up to the

user as timed out.

Published a review request with the broker running but no workers. Saw the new

status update display as "starting...". Fired up the worker node and saw it

process the task, updating the status update to "running..." and then

finished.