Better support for Jobs with missing paths

[gperciva]

There doesn't appear to be any way to recover from a Job from missing paths. (i.e. if you move to a new computer)

• The MainWindow thoughtfully warns you that there's missing paths... but doesn't actually tell you what they are.
• The tree view in the Jobs->Info window doesn't show the missing paths, so there's no way to remove them (even if MainWindow gave a proper warning).

At the moment, the only option I see is to remove the old Job and make a new one.