patch to make media files more resilient to disk full conditions

[Mark Harvey]

A disk full condition while writing to virtual media will most likely result in a corrupted media condition.

This patch attempts to roll back any partial writes:
0001-write-error-Make-backend-format-more-resilient-again.patch

I'm open to comments, testers etc..

I've performed a few sanity checks with NetBackup and 100% positive outcomes so far.