dead code/bug in error-handling in mutable.layout.MDMFSlotWriteProxy._write

[zooko]

While investigating #2342, Brian and I noticed that this code​ appears to be buggy. It is calling addCallback(), and then the callback function is checking if the arg is a Failure, but Failure's would never get sent to a callback function, only to an errback function, which is not handled (here).

[zooko]

To close this ticket, run the unit tests with code coverage, inspect the code coverage results around these lines here, then remove the handling of Failure inside _result() and re-run the complete unit tests, with code coverage, and examine the code coverage results again. I've already looked for whether there is an unhandled error path here, and I think I concluded that any error here gets handled in this code, but you should perhaps keep your wits about you and try to double-check that belief as you go.