MODE-2298 Updated sequencers and text extractors to better handle the case when a repository has been shut down and there's still work being processed

This message doesn't seem to convey the message I was thinking of when I logged MODE-2298. There really are several points we want to make clear:

1. The sequencing operation terminated abnormally and will not complete
2. This was entirely because the repository has been shutdown while the sequencing operation was running
3. The underlying reason may or may not be meaningful

So how about something like the following?

> The repository has been shut down while sequencing '{path}', resulting in premature termination of the sequencing operation with underlying cause: {message}