DFU file publish (onDFUFilePublish) can fail with follow-on error obscuring original cause of failure

On DFU publish, if there's a exception failure (e.g. during attach), then the clearup process may hit another exception and report that follow-up exception instead of the original.

A particular case of this is when attach() fails, and the wind-up code tries to detach the newfile, which in turns fails because it is illegal to call detach on a file that is not yet attached.
Fix this related issue also.