Improve progress monitor for jobs

@devoncarew

Improves upon #2688:
* uses the progress monitor's title in the status message, not the job's name
* formats the progress value as "x%", "x of y", "x.y" instead of always "x.y"

Also fixes a checked mode exception and adds an assertion.
Oops, bad naming: changed to `_progressAsString`. I don't know about default text: I think it can potentially mask errors.