store a status string in the storage obligations

When a storage obligation is completed, a status string is added to the obligation stored on disk. This allows hosts to view their contracts historically and see why they failed in the event of a failure, which will help with debugging/troubleshooting

I'm not entirely certain that a string is the best move. An alternate option has us using error codes. A string is nice because it lets you see the actual meaning of the error directly in the database. But if we ever change the errors, the errors could be lost from the codebase.

I think a string is more likely to make sense in the long run, but it does mean you could be left with a string that you are unable to grep.